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Nowadays before starting a reflink operation we do this:
1) Take the VFS lock of the inodes in exclusive mode (a rw semaphore);
2) Take the mmap lock of the inodes (struct btrfs_inode::i_mmap_lock);
3) Flush all delalloc in the source and target ranges;
4) Wait for all ordered extents in the source and target ranges to
complete;
5) Lock the source and destination ranges in the inodes' io trees.
In step 5 we lock the source range because:
1) We needed to serialize against mmap writes, but that is not needed
anymore because nowadays we do that through the inode's i_mmap_lock
(step 2). This happens since commit 8c99516a8cdd ("btrfs: exclude mmaps
while doing remap");
2) To serialize against a concurrent relocation and avoid generating
a delayed ref for an extent that was just dropped by relocation, see
commit d8b552424210 ("Btrfs: fix race between reflink/dedupe and
relocation").
Locking the source range however blocks any concurrent reads for that
range and makes test case generic/733 fail.
So instead of locking the source range during reflinks, make relocation
read lock the inode's i_mmap_lock, so that it serializes with a concurrent
reflink while still able to run concurrently with mmap writes and allow
concurrent reads too.
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
5d6f0e9890