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Joao Martins 11aad2631b mm/hugetlb_vmemmap: remap head page to newly allocated page
Today with `hugetlb_free_vmemmap=on` the struct page memory that is freed
back to page allocator is as following: for a 2M hugetlb page it will reuse
the first 4K vmemmap page to remap the remaining 7 vmemmap pages, and for a
1G hugetlb it will remap the remaining 4095 vmemmap pages. Essentially,
that means that it breaks the first 4K of a potentially contiguous chunk of
memory of 32K (for 2M hugetlb pages) or 16M (for 1G hugetlb pages). For
this reason the memory that it's free back to page allocator cannot be used
for hugetlb to allocate huge pages of the same size, but rather only of a
smaller huge page size:

Trying to assign a 64G node to hugetlb (on a 128G 2node guest, each node
having 64G):

* Before allocation:
Free pages count per migrate type at order       0      1      2      3
4      5      6      7      8      9     10
...
Node    0, zone   Normal, type      Movable    340    100     32     15
1      2      0      0      0      1  15558

$ echo 32768 > /sys/devices/system/node/node0/hugepages/hugepages-2048kB/nr_hugepages
$ cat /sys/devices/system/node/node0/hugepages/hugepages-2048kB/nr_hugepages
 31987

* After:

Node    0, zone   Normal, type      Movable  30893  32006  31515      7
0      0      0      0      0      0      0

Notice how the memory freed back are put back into 4K / 8K / 16K page
pools. And it allocates a total of 31987 pages (63974M).

To fix this behaviour rather than remapping second vmemmap page (thus
breaking the contiguous block of memory backing the struct pages)
repopulate the first vmemmap page with a new one. We allocate and copy
from the currently mapped vmemmap page, and then remap it later on.
The same algorithm works if there's a pre initialized walk::reuse_page
and the head page doesn't need to be skipped and instead we remap it
when the @addr being changed is the @reuse_addr.

The new head page is allocated in vmemmap_remap_free() given that on
restore there's no need for functional change. Note that, because right
now one hugepage is remapped at a time, thus only one free 4K page at a
time is needed to remap the head page. Should it fail to allocate said
new page, it reuses the one that's already mapped just like before. As a
result, for every 64G of contiguous hugepages it can give back 1G more
of contiguous memory per 64G, while needing in total 128M new 4K pages
(for 2M hugetlb) or 256k (for 1G hugetlb).

After the changes, try to assign a 64G node to hugetlb (on a 128G 2node
guest, each node with 64G):

* Before allocation
Free pages count per migrate type at order       0      1      2      3
4      5      6      7      8      9     10
...
Node    0, zone   Normal, type      Movable      1      1      1      0
0      1      0      0      1      1  15564

$ echo 32768  > /sys/devices/system/node/node0/hugepages/hugepages-2048kB/nr_hugepages
$ cat /sys/devices/system/node/node0/hugepages/hugepages-2048kB/nr_hugepages
32394

* After:

Node    0, zone   Normal, type      Movable      0     50     97    108
96     81     70     46     18      0      0

In the example above, 407 more hugeltb 2M pages are allocated i.e. 814M out
of the 32394 (64788M) allocated. So the memory freed back is indeed being
used back in hugetlb and there's no massive order-0..order-2 pages
accumulated unused.

[joao.m.martins@oracle.com: v3]
  Link: https://lkml.kernel.org/r/20221109200623.96867-1-joao.m.martins@oracle.com
[joao.m.martins@oracle.com: add smp_wmb() to ensure page contents are visible prior to PTE write]
  Link: https://lkml.kernel.org/r/20221110121214.6297-1-joao.m.martins@oracle.com
Link: https://lkml.kernel.org/r/20221107153922.77094-1-joao.m.martins@oracle.com
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
2022-11-30 15:58:47 -08:00
arch Merge branch 'mm-hotfixes-stable' into mm-stable 2022-11-30 14:58:42 -08:00
block block-6.1-2022-11-05 2022-11-05 09:02:28 -07:00
certs certs: make system keyring depend on built-in x509 parser 2022-09-24 04:31:18 +09:00
crypto treewide: use get_random_bytes() when possible 2022-10-11 17:42:58 -06:00
Documentation mm,thp,rmap: lock_compound_mapcounts() on THP mapcounts 2022-11-30 15:58:47 -08:00
drivers Merge branch 'mm-hotfixes-stable' into mm-stable 2022-11-30 14:58:42 -08:00
fs Merge branch 'mm-hotfixes-stable' into mm-stable 2022-11-30 14:58:42 -08:00
include mm,thp,rmap: lock_compound_mapcounts() on THP mapcounts 2022-11-30 15:58:47 -08:00
init init: Kconfig: fix spelling mistake "satify" -> "satisfy" 2022-10-20 21:27:22 -07:00
io_uring io_uring: unlock if __io_run_local_work locked inside 2022-10-27 09:52:12 -06:00
ipc ipc/shm: call underlying open/close vm_ops 2022-11-22 18:50:42 -08:00
kernel percpu_counter: add percpu_counter_sum_all interface 2022-11-30 15:58:40 -08:00
lib maple_tree: mte_set_full() and mte_clear_full() clang-analyzer clean up 2022-11-30 15:58:41 -08:00
LICENSES LICENSES/LGPL-2.1: Add LGPL-2.1-or-later as valid identifiers 2021-12-16 14:33:10 +01:00
mm mm/hugetlb_vmemmap: remap head page to newly allocated page 2022-11-30 15:58:47 -08:00
net Networking fixes for 6.1-rc4, including fixes from bluetooth and 2022-11-03 10:51:59 -07:00
rust Kbuild: add Rust support 2022-09-28 09:02:20 +02:00
samples VFIO updates for v6.1-rc1 2022-10-12 14:46:48 -07:00
scripts kconfig: fix segmentation fault in menuconfig search 2022-11-02 17:32:05 +09:00
security lsm/stable-6.1 PR 20221031 2022-10-31 12:09:42 -07:00
sound ALSA: aoa: Fix I2S device accounting 2022-10-27 08:53:08 +02:00
tools selftests/damon: test non-context inputs to rm_contexts file 2022-11-30 15:58:47 -08:00
usr usr/gen_init_cpio.c: remove unnecessary -1 values from int file 2022-10-03 14:21:44 -07:00
virt Merge tag 'kvmarm-fixes-6.1-3' of git://git.kernel.org/pub/scm/linux/kernel/git/kvmarm/kvmarm into HEAD 2022-11-06 03:30:49 -05:00
.clang-format PCI/DOE: Add DOE mailbox support functions 2022-07-19 15:38:04 -07:00
.cocciconfig
.get_maintainer.ignore get_maintainer: add Alan to .get_maintainer.ignore 2022-08-20 15:17:44 -07:00
.gitattributes
.gitignore Kbuild: add Rust support 2022-09-28 09:02:20 +02:00
.mailmap mailmap: update email address for Satya Priya 2022-11-22 18:50:43 -08:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS drm for 5.20/6.0 2022-08-03 19:52:08 -07:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS Merge branch 'mm-hotfixes-stable' into mm-stable 2022-11-30 14:58:42 -08:00
Makefile Linux 6.1-rc4 2022-11-06 15:07:11 -08:00
README

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