On 2019/5/4 上午5:56, Zygo Blaxell wrote: > On Fri, May 03, 2019 at 09:08:52AM +0800, Qu Wenruo wrote: >> [BUG] >> The following command can lead to unexpected data COW: >> >> #!/bin/bash >> >> dev=/dev/test/test >> mnt=/mnt/btrfs >> >> mkfs.btrfs -f $dev -b 1G > /dev/null >> mount $dev $mnt -o nospace_cache >> >> xfs_io -f -c "falloc 8k 24k" -c "pwrite 12k 8k" $mnt/file1 >> xfs_io -c "reflink $mnt/file1 8k 0 4k" $mnt/file1 >> umount $dev >> >> The result extent will be >> >> item 7 key (257 EXTENT_DATA 4096) itemoff 15760 itemsize 53 >> generation 6 type 2 (prealloc) >> prealloc data disk byte 13631488 nr 28672 >> item 8 key (257 EXTENT_DATA 12288) itemoff 15707 itemsize 53 >> generation 6 type 1 (regular) >> extent data disk byte 13660160 nr 12288 <<< COW >> item 9 key (257 EXTENT_DATA 24576) itemoff 15654 itemsize 53 >> generation 6 type 2 (prealloc) >> prealloc data disk byte 13631488 nr 28672 >> >> Currently we always reserve space even for NOCOW buffered write, thus >> under most case it shouldn't cause anything wrong even we fall back to >> COW. >> >> However when we're out of data space, we fall back to skip data space if >> we can do NOCOW write. >> >> If such behavior happens under that case, we could hit the following >> problems: >> - data space bytes_may_use underflow >> This will cause kernel warning. >> >> - ENOSPC at delalloc time >> This will lead to transaction abort and fs forced to RO. >> >> [CAUSE] >> This is due to the fact that btrfs can only do extent level share check. >> >> Btrfs can only tell if an extent is shared, no matter if only part of the >> extent is shared or not. >> >> So for above script we have: >> - fallocate >> - buffered write >> If we don't have enough data space, we fall back to NOCOW check. >> At this timming, the extent is not shared, we can skip data >> reservation. >> - reflink >> Now part of the large preallocated extent is shared. >> - delalloc kicks in >> For the NOCOW range, as the preallocated extent is shared, we need >> to fall back to COW. >> >> [WORKAROUND] >> The workaround is to ensure any buffered write in the related extents >> (not the reflink source range) get flushed before reflink. >> >> However it's pretty expensive to do a comprehensive check. >> In the reproducer, the reflink source is just a part of a larger >> preallocated extent, we need to flush all buffered write of that extent >> before reflink. >> Such backward search can be complex and we may not get much benefit from >> it. >> >> So this patch will just try to flush the whole inode before reflink. > > Does that mean that if a large file is being written and deduped > simultaneously, that the dedupes would now trigger flushes over the > entire file? That seems like it could be slow. Yes, also my reason for RFC. But it shouldn't be that heavy, as after the first dedupe/reflink, most IO should be flushed back, later dedupe has much less work to do. > > e.g. if the file is a big VM image file and it is used src and for dedupe > (which is quite common in VM image files), we'd be hammering the disk > with writes similar to hitting it with fsync() in a tight loop? The original behavior also flush the target and source range, so we're not completely creating some new overhead. Thanks, Qu > >> Signed-off-by: Qu Wenruo <wqu@xxxxxxxx> >> --- >> Reason for RFC: >> Flushing an inode just because it's a reflink source is definitely >> overkilling, but I don't have any better way to handle it. >> >> Any comment on this is welcomed. >> --- >> fs/btrfs/ioctl.c | 22 ++++++++++++++++++++++ >> 1 file changed, 22 insertions(+) >> >> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c >> index 7755b503b348..8caa0edb6fbf 100644 >> --- a/fs/btrfs/ioctl.c >> +++ b/fs/btrfs/ioctl.c >> @@ -3930,6 +3930,28 @@ static noinline int btrfs_clone_files(struct file *file, struct file *file_src, >> return ret; >> } >> >> + /* >> + * Workaround to make sure NOCOW buffered write reach disk as NOCOW. >> + * >> + * Due to the limit of btrfs extent tree design, we can only have >> + * extent level share view. Any part of an extent is shared then the >> + * whole extent is shared and any write into that extent needs to fall >> + * back to COW. >> + * >> + * NOCOW buffered write without data space reserved could to lead to >> + * either data space bytes_may_use underflow (kernel warning) or ENOSPC >> + * at delalloc time (transaction abort). >> + * >> + * Here we take a shortcut by flush the whole inode. We could do better >> + * by finding all extents in that range and flush the space referring >> + * all those extents. >> + * But that's too complex for such corner case. >> + */ >> + filemap_flush(src->i_mapping); >> + if (test_bit(BTRFS_INODE_HAS_ASYNC_EXTENT, >> + &BTRFS_I(src)->runtime_flags)) >> + filemap_flush(src->i_mapping); >> + >> /* >> * Lock destination range to serialize with concurrent readpages() and >> * source range to serialize with relocation. >> -- >> 2.21.0 >>
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