btrfs: introduce size class to block group allocator
The aim of this patch is to reduce the fragmentation of block groups under certain unhappy workloads. It is particularly effective when the size of extents correlates with their lifetime, which is something we have observed causing fragmentation in the fleet at Meta. This patch categorizes extents into size classes: - x < 128KiB: "small" - 128KiB < x < 8MiB: "medium" - x > 8MiB: "large" and as much as possible reduces allocations of extents into block groups that don't match the size class. This takes advantage of any (possible) correlation between size and lifetime and also leaves behind predictable re-usable gaps when extents are freed; small writes don't gum up bigger holes. Size classes are implemented in the following way: - Mark each new block group with a size class of the first allocation that goes into it. - Add two new passes to ffe: "unset size class" and "wrong size class". First, try only matching block groups, then try unset ones, then allow allocation of new ones, and finally allow mismatched block groups. - Filtering is done just by skipping inappropriate ones, there is no special size class indexing. Other solutions I considered were: - A best fit allocator with an rb-tree. This worked well, as small writes didn't leak big holes from large freed extents, but led to regressions in ffe and write performance due to lock contention on the rb-tree with every allocation possibly updating it in parallel. Perhaps something clever could be done to do the updates in the background while being "right enough". - A fixed size "working set". This prevents freeing an extent drastically changing where writes currently land, and seems like a good option too. Doesn't take advantage of size in any way. - The same size class idea, but implemented with xarray marks. This turned out to be slower than looping the linked list and skipping wrong block groups, and is also less flexible since we must have only 3 size classes (max #marks). With the current approach we can have as many as we like. Performance testing was done via: https://github.com/josefbacik/fsperf Of particular relevance are the new fragmentation specific tests. A brief summary of the testing results: - Neutral results on existing tests. There are some minor regressions and improvements here and there, but nothing that truly stands out as notable. - Improvement on new tests where size class and extent lifetime are correlated. Fragmentation in these cases is completely eliminated and write performance is generally a little better. There is also significant improvement where extent sizes are just a bit larger than the size class boundaries. - Regression on one new tests: where the allocations are sized intentionally a hair under the borders of the size classes. Results are neutral on the test that intentionally attacks this new scheme by mixing extent size and lifetime. The full dump of the performance results can be found here: https://bur.io/fsperf/size-class-2022-11-15.txt (there are ANSI escape codes, so best to curl and view in terminal) Here is a snippet from the full results for a new test which mixes buffered writes appending to a long lived set of files and large short lived fallocates: bufferedappendvsfallocate results metric baseline current stdev diff ====================================================================================== avg_commit_ms 31.13 29.20 2.67 -6.22% bg_count 14 15.60 0 11.43% commits 11.10 12.20 0.32 9.91% elapsed 27.30 26.40 2.98 -3.30% end_state_mount_ns 11122551.90 10635118.90 851143.04 -4.38% end_state_umount_ns 1.36e+09 1.35e+09 12248056.65 -1.07% find_free_extent_calls 116244.30 114354.30 964.56 -1.63% find_free_extent_ns_max 599507.20 1047168.20 103337.08 74.67% find_free_extent_ns_mean 3607.19 3672.11 101.20 1.80% find_free_extent_ns_min 500 512 6.67 2.40% find_free_extent_ns_p50 2848 2876 37.65 0.98% find_free_extent_ns_p95 4916 5000 75.45 1.71% find_free_extent_ns_p99 20734.49 20920.48 1670.93 0.90% frag_pct_max 61.67 0 8.05 -100.00% frag_pct_mean 43.59 0 6.10 -100.00% frag_pct_min 25.91 0 16.60 -100.00% frag_pct_p50 42.53 0 7.25 -100.00% frag_pct_p95 61.67 0 8.05 -100.00% frag_pct_p99 61.67 0 8.05 -100.00% fragmented_bg_count 6.10 0 1.45 -100.00% max_commit_ms 49.80 46 5.37 -7.63% sys_cpu 2.59 2.62 0.29 1.39% write_bw_bytes 1.62e+08 1.68e+08 17975843.50 3.23% write_clat_ns_mean 57426.39 54475.95 2292.72 -5.14% write_clat_ns_p50 46950.40 42905.60 2101.35 -8.62% write_clat_ns_p99 148070.40 143769.60 2115.17 -2.90% write_io_kbytes 4194304 4194304 0 0.00% write_iops 2476.15 2556.10 274.29 3.23% write_lat_ns_max 2101667.60 2251129.50 370556.59 7.11% write_lat_ns_mean 59374.91 55682.00 2523.09 -6.22% write_lat_ns_min 17353.10 16250 1646.08 -6.36% There are some mixed improvements/regressions in most metrics along with an elimination of fragmentation in this workload. On the balance, the drastic 1->0 improvement in the happy cases seems worth the mix of regressions and improvements we do observe. Some considerations for future work: - Experimenting with more size classes - More hinting/search ordering work to approximate a best-fit allocator Signed-off-by: Boris Burkov <boris@bur.io> Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Boris Burkov
David Sterba
parent
854c2f365d
commit
52bb7a2166
@ -1,5 +1,6 @@
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// SPDX-License-Identifier: GPL-2.0
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#include <linux/sizes.h>
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#include <linux/list_sort.h>
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#include "misc.h"
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#include "ctree.h"
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@ -3379,6 +3380,7 @@ int btrfs_update_block_group(struct btrfs_trans_handle *trans,
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cache->space_info->disk_used -= num_bytes * factor;
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reclaim = should_reclaim_block_group(cache, num_bytes);
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spin_unlock(&cache->lock);
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spin_unlock(&cache->space_info->lock);
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@ -3433,32 +3435,42 @@ int btrfs_update_block_group(struct btrfs_trans_handle *trans,
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* reservation and return -EAGAIN, otherwise this function always succeeds.
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*/
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int btrfs_add_reserved_bytes(struct btrfs_block_group *cache,
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u64 ram_bytes, u64 num_bytes, int delalloc)
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u64 ram_bytes, u64 num_bytes, int delalloc,
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bool force_wrong_size_class)
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{
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struct btrfs_space_info *space_info = cache->space_info;
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enum btrfs_block_group_size_class size_class;
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int ret = 0;
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spin_lock(&space_info->lock);
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spin_lock(&cache->lock);
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if (cache->ro) {
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ret = -EAGAIN;
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} else {
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cache->reserved += num_bytes;
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space_info->bytes_reserved += num_bytes;
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trace_btrfs_space_reservation(cache->fs_info, "space_info",
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space_info->flags, num_bytes, 1);
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btrfs_space_info_update_bytes_may_use(cache->fs_info,
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space_info, -ram_bytes);
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if (delalloc)
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cache->delalloc_bytes += num_bytes;
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/*
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* Compression can use less space than we reserved, so wake
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* tickets if that happens
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*/
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if (num_bytes < ram_bytes)
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btrfs_try_granting_tickets(cache->fs_info, space_info);
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goto out;
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}
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if (btrfs_is_block_group_data_only(cache)) {
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size_class = btrfs_calc_block_group_size_class(num_bytes);
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ret = btrfs_use_block_group_size_class(cache, size_class, force_wrong_size_class);
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if (ret)
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goto out;
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}
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cache->reserved += num_bytes;
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space_info->bytes_reserved += num_bytes;
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trace_btrfs_space_reservation(cache->fs_info, "space_info",
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space_info->flags, num_bytes, 1);
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btrfs_space_info_update_bytes_may_use(cache->fs_info,
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space_info, -ram_bytes);
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if (delalloc)
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cache->delalloc_bytes += num_bytes;
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/*
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* Compression can use less space than we reserved, so wake tickets if
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* that happens.
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*/
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if (num_bytes < ram_bytes)
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btrfs_try_granting_tickets(cache->fs_info, space_info);
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out:
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spin_unlock(&cache->lock);
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spin_unlock(&space_info->lock);
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return ret;
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@ -4218,3 +4230,64 @@ void btrfs_dec_block_group_swap_extents(struct btrfs_block_group *bg, int amount
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bg->swap_extents -= amount;
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spin_unlock(&bg->lock);
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}
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enum btrfs_block_group_size_class btrfs_calc_block_group_size_class(u64 size)
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{
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if (size <= SZ_128K)
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return BTRFS_BG_SZ_SMALL;
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if (size <= SZ_8M)
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return BTRFS_BG_SZ_MEDIUM;
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return BTRFS_BG_SZ_LARGE;
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}
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/*
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* Handle a block group allocating an extent in a size class
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*
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* @bg: The block group we allocated in.
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* @size_class: The size class of the allocation.
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* @force_wrong_size_class: Whether we are desperate enough to allow
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* mismatched size classes.
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*
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* Returns: 0 if the size class was valid for this block_group, -EAGAIN in the
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* case of a race that leads to the wrong size class without
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* force_wrong_size_class set.
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*
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* find_free_extent will skip block groups with a mismatched size class until
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* it really needs to avoid ENOSPC. In that case it will set
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* force_wrong_size_class. However, if a block group is newly allocated and
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* doesn't yet have a size class, then it is possible for two allocations of
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* different sizes to race and both try to use it. The loser is caught here and
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* has to retry.
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*/
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int btrfs_use_block_group_size_class(struct btrfs_block_group *bg,
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enum btrfs_block_group_size_class size_class,
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bool force_wrong_size_class)
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{
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ASSERT(size_class != BTRFS_BG_SZ_NONE);
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/* The new allocation is in the right size class, do nothing */
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if (bg->size_class == size_class)
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return 0;
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/*
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* The new allocation is in a mismatched size class.
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* This means one of two things:
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*
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* 1. Two tasks in find_free_extent for different size_classes raced
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* and hit the same empty block_group. Make the loser try again.
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* 2. A call to find_free_extent got desperate enough to set
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* 'force_wrong_slab'. Don't change the size_class, but allow the
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* allocation.
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*/
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if (bg->size_class != BTRFS_BG_SZ_NONE) {
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if (force_wrong_size_class)
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return 0;
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return -EAGAIN;
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}
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/*
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* The happy new block group case: the new allocation is the first
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* one in the block_group so we set size_class.
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*/
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bg->size_class = size_class;
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return 0;
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}
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@ -12,6 +12,17 @@ enum btrfs_disk_cache_state {
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BTRFS_DC_SETUP,
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};
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enum btrfs_block_group_size_class {
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/* Unset */
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BTRFS_BG_SZ_NONE,
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/* 0 < size <= 128K */
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BTRFS_BG_SZ_SMALL,
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/* 128K < size <= 8M */
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BTRFS_BG_SZ_MEDIUM,
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/* 8M < size < BG_LENGTH */
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BTRFS_BG_SZ_LARGE,
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};
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/*
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* This describes the state of the block_group for async discard. This is due
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* to the two pass nature of it where extent discarding is prioritized over
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@ -233,6 +244,7 @@ struct btrfs_block_group {
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struct list_head active_bg_list;
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struct work_struct zone_finish_work;
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struct extent_buffer *last_eb;
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enum btrfs_block_group_size_class size_class;
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};
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static inline u64 btrfs_block_group_end(struct btrfs_block_group *block_group)
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@ -302,7 +314,8 @@ int btrfs_setup_space_cache(struct btrfs_trans_handle *trans);
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int btrfs_update_block_group(struct btrfs_trans_handle *trans,
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u64 bytenr, u64 num_bytes, bool alloc);
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int btrfs_add_reserved_bytes(struct btrfs_block_group *cache,
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u64 ram_bytes, u64 num_bytes, int delalloc);
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u64 ram_bytes, u64 num_bytes, int delalloc,
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bool force_wrong_size_class);
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void btrfs_free_reserved_bytes(struct btrfs_block_group *cache,
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u64 num_bytes, int delalloc);
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int btrfs_chunk_alloc(struct btrfs_trans_handle *trans, u64 flags,
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@ -346,4 +359,9 @@ void btrfs_unfreeze_block_group(struct btrfs_block_group *cache);
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bool btrfs_inc_block_group_swap_extents(struct btrfs_block_group *bg);
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void btrfs_dec_block_group_swap_extents(struct btrfs_block_group *bg, int amount);
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enum btrfs_block_group_size_class btrfs_calc_block_group_size_class(u64 size);
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int btrfs_use_block_group_size_class(struct btrfs_block_group *bg,
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enum btrfs_block_group_size_class size_class,
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bool force_wrong_size_class);
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#endif /* BTRFS_BLOCK_GROUP_H */
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@ -3385,7 +3385,9 @@ int btrfs_free_extent(struct btrfs_trans_handle *trans, struct btrfs_ref *ref)
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enum btrfs_loop_type {
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LOOP_CACHING_NOWAIT,
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LOOP_CACHING_WAIT,
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LOOP_UNSET_SIZE_CLASS,
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LOOP_ALLOC_CHUNK,
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LOOP_WRONG_SIZE_CLASS,
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LOOP_NO_EMPTY_SIZE,
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};
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@ -3950,24 +3952,6 @@ static int can_allocate_chunk(struct btrfs_fs_info *fs_info,
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}
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}
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static int chunk_allocation_failed(struct find_free_extent_ctl *ffe_ctl)
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{
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switch (ffe_ctl->policy) {
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case BTRFS_EXTENT_ALLOC_CLUSTERED:
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/*
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* If we can't allocate a new chunk we've already looped through
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* at least once, move on to the NO_EMPTY_SIZE case.
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*/
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ffe_ctl->loop = LOOP_NO_EMPTY_SIZE;
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return 0;
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case BTRFS_EXTENT_ALLOC_ZONED:
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/* Give up here */
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return -ENOSPC;
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default:
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BUG();
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}
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}
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/*
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* Return >0 means caller needs to re-search for free extent
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* Return 0 means we have the needed free extent.
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@ -4001,31 +3985,28 @@ static int find_free_extent_update_loop(struct btrfs_fs_info *fs_info,
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* LOOP_CACHING_NOWAIT, search partially cached block groups, kicking
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* caching kthreads as we move along
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* LOOP_CACHING_WAIT, search everything, and wait if our bg is caching
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* LOOP_UNSET_SIZE_CLASS, allow unset size class
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* LOOP_ALLOC_CHUNK, force a chunk allocation and try again
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* LOOP_NO_EMPTY_SIZE, set empty_size and empty_cluster to 0 and try
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* again
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*/
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if (ffe_ctl->loop < LOOP_NO_EMPTY_SIZE) {
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ffe_ctl->index = 0;
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if (ffe_ctl->loop == LOOP_CACHING_NOWAIT) {
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/*
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* We want to skip the LOOP_CACHING_WAIT step if we
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* don't have any uncached bgs and we've already done a
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* full search through.
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*/
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if (ffe_ctl->orig_have_caching_bg || !full_search)
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ffe_ctl->loop = LOOP_CACHING_WAIT;
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else
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ffe_ctl->loop = LOOP_ALLOC_CHUNK;
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} else {
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/*
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* We want to skip the LOOP_CACHING_WAIT step if we don't have
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* any uncached bgs and we've already done a full search
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* through.
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*/
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if (ffe_ctl->loop == LOOP_CACHING_NOWAIT &&
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(!ffe_ctl->orig_have_caching_bg && full_search))
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ffe_ctl->loop++;
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}
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ffe_ctl->loop++;
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if (ffe_ctl->loop == LOOP_ALLOC_CHUNK) {
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struct btrfs_trans_handle *trans;
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int exist = 0;
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/*Check if allocation policy allows to create a new chunk */
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/* Check if allocation policy allows to create a new chunk */
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ret = can_allocate_chunk(fs_info, ffe_ctl);
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if (ret)
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return ret;
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@ -4045,8 +4026,10 @@ static int find_free_extent_update_loop(struct btrfs_fs_info *fs_info,
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CHUNK_ALLOC_FORCE_FOR_EXTENT);
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/* Do not bail out on ENOSPC since we can do more. */
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if (ret == -ENOSPC)
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ret = chunk_allocation_failed(ffe_ctl);
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if (ret == -ENOSPC) {
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ret = 0;
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ffe_ctl->loop++;
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}
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else if (ret < 0)
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btrfs_abort_transaction(trans, ret);
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else
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@ -4076,6 +4059,21 @@ static int find_free_extent_update_loop(struct btrfs_fs_info *fs_info,
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return -ENOSPC;
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}
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static bool find_free_extent_check_size_class(struct find_free_extent_ctl *ffe_ctl,
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struct btrfs_block_group *bg)
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{
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if (ffe_ctl->policy == BTRFS_EXTENT_ALLOC_ZONED)
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return true;
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if (!btrfs_is_block_group_data_only(bg))
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return true;
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if (ffe_ctl->loop >= LOOP_WRONG_SIZE_CLASS)
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return true;
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if (ffe_ctl->loop >= LOOP_UNSET_SIZE_CLASS &&
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bg->size_class == BTRFS_BG_SZ_NONE)
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return true;
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return ffe_ctl->size_class == bg->size_class;
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}
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static int prepare_allocation_clustered(struct btrfs_fs_info *fs_info,
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struct find_free_extent_ctl *ffe_ctl,
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struct btrfs_space_info *space_info,
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@ -4210,6 +4208,7 @@ static noinline int find_free_extent(struct btrfs_root *root,
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ffe_ctl->total_free_space = 0;
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ffe_ctl->found_offset = 0;
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ffe_ctl->policy = BTRFS_EXTENT_ALLOC_CLUSTERED;
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ffe_ctl->size_class = btrfs_calc_block_group_size_class(ffe_ctl->num_bytes);
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if (btrfs_is_zoned(fs_info))
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ffe_ctl->policy = BTRFS_EXTENT_ALLOC_ZONED;
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@ -4346,6 +4345,9 @@ have_block_group:
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if (unlikely(block_group->cached == BTRFS_CACHE_ERROR))
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goto loop;
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if (!find_free_extent_check_size_class(ffe_ctl, block_group))
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goto loop;
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bg_ret = NULL;
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ret = do_allocation(block_group, ffe_ctl, &bg_ret);
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if (ret == 0) {
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@ -4380,7 +4382,8 @@ have_block_group:
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ret = btrfs_add_reserved_bytes(block_group, ffe_ctl->ram_bytes,
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ffe_ctl->num_bytes,
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ffe_ctl->delalloc);
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ffe_ctl->delalloc,
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ffe_ctl->loop >= LOOP_WRONG_SIZE_CLASS);
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if (ret == -EAGAIN) {
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btrfs_add_free_space_unused(block_group,
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ffe_ctl->found_offset,
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@ -79,6 +79,9 @@ struct find_free_extent_ctl {
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/* Whether or not the allocator is currently following a hint */
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bool hinted;
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/* Size class of block groups to prefer in early loops */
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enum btrfs_block_group_size_class size_class;
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};
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enum btrfs_inline_ref_type {
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@ -1349,28 +1349,33 @@ DECLARE_EVENT_CLASS(btrfs__reserve_extent,
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TP_STRUCT__entry_btrfs(
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__field( u64, bg_objectid )
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||||
__field( u64, flags )
|
||||
__field( int, bg_size_class )
|
||||
__field( u64, start )
|
||||
__field( u64, len )
|
||||
__field( u64, loop )
|
||||
__field( bool, hinted )
|
||||
__field( int, size_class )
|
||||
),
|
||||
|
||||
TP_fast_assign_btrfs(block_group->fs_info,
|
||||
__entry->bg_objectid = block_group->start;
|
||||
__entry->flags = block_group->flags;
|
||||
__entry->bg_size_class = block_group->size_class;
|
||||
__entry->start = ffe_ctl->search_start;
|
||||
__entry->len = ffe_ctl->num_bytes;
|
||||
__entry->loop = ffe_ctl->loop;
|
||||
__entry->hinted = ffe_ctl->hinted;
|
||||
__entry->size_class = ffe_ctl->size_class;
|
||||
),
|
||||
|
||||
TP_printk_btrfs(
|
||||
"root=%llu(%s) block_group=%llu flags=%llu(%s) start=%llu len=%llu loop=%llu hinted=%d",
|
||||
"root=%llu(%s) block_group=%llu flags=%llu(%s) bg_size_class=%d start=%llu len=%llu loop=%llu hinted=%d size_class=%d",
|
||||
show_root_type(BTRFS_EXTENT_TREE_OBJECTID),
|
||||
__entry->bg_objectid,
|
||||
__entry->flags, __print_flags((unsigned long)__entry->flags,
|
||||
"|", BTRFS_GROUP_FLAGS),
|
||||
__entry->start, __entry->len, __entry->loop, __entry->hinted)
|
||||
__entry->bg_size_class, __entry->start, __entry->len,
|
||||
__entry->loop, __entry->hinted, __entry->size_class)
|
||||
);
|
||||
|
||||
DEFINE_EVENT(btrfs__reserve_extent, btrfs_reserve_extent,
|
||||
|
||||
Loading…
Reference in New Issue
Block a user