On 15.05.2018 10:36, Qu Wenruo wrote:
> As btrfs(5) specified:
>
> Note
> If nodatacow or nodatasum are enabled, compression is disabled.
>
> If NODATASUM or NODATACOW set, we should not compress the extent.
>
> Normally NODATACOW is detected properly in run_delalloc_range() so
> compression won't happen for NODATACOW.
>
> However for NODATASUM we don't have any check, and it can cause
> compressed extent without csum pretty easily, just by:
> ------
> mkfs.btrfs -f $dev
> mount $dev $mnt -o nodatasum
> touch $mnt/foobar
> mount -o remount,datasum,compress $mnt
> xfs_io -f -c "pwrite 0 128K" $mnt/foobar
> ------
>
> And in fact, we have bug report about corrupted compressed extent
> without proper data checksum so even RAID1 can't recover the corruption.
> (https://bugzilla.kernel.org/show_bug.cgi?id=199707)
>
> Running compression without proper checksum could cause more damage when
> corruption happens, so there is no need to allow compression for
> NODATACSUM.
>
> Reported-by: James Harvey <[email protected]>
> Signed-off-by: Qu Wenruo <[email protected]>
> ---
> fs/btrfs/inode.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index d241285a0d2a..dbef3f404559 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -396,6 +396,14 @@ static inline int inode_need_compress(struct inode
> *inode, u64 start, u64 end)
> {
> struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
>
> + /*
> + * Btrfs doesn't support compression without csum or CoW.
> + * This should have the highest priority.
> + */
> + if (BTRFS_I(inode)->flags & BTRFS_INODE_NODATACOW ||
> + BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM)
> + return 0;
> +
How is this not buggy, given that if inode_need_compress as called from
compress_file_range will return zero, meaning we jump to cont: label.
Then in the case of an inline extent we can execute :
ret = cow_file_range_inline(inode, start, end,
total_compressed,
compress_type, pages);
where compress_type would have been set at the beginning of the
function unconditionally to fs_info->compress_type.
For non-inline extents I guess we are ok, given that will_compress
will not be set. However, this code is rather messy and I'm not sure
it's well defined what's going to happen in this case with inline extents.
OTOH, I think there is something fundamentally wrong in calling
inode_need_compress in compress_file_range. I.e they work at different
abstractions. IMO compress_file_range should only be called if we know
we have to compress the range.
So looking around the code in run_delalloc_range (the only function
which calls cow_file_range_async) we already have :
} else if (!inode_need_compress(inode, start, end)) {
ret = cow_file_range(inode, locked_page, start, end, end,
page_started, nr_written, 1, NULL);
and in the else branch we have the cow_file_range_async. So the code
is sort of half-way there to actually decoupling compression checking from
performing the actual compression.
> /* force compress */
> if (btrfs_test_opt(fs_info, FORCE_COMPRESS))
> return 1;
One more thing, in inode_need_compress shouldn't the inode specific
checks come first something like :
static inline int inode_need_compress(struct inode *inode, u64 start, u64 end)
{
struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
/* defrag ioctl */
if (BTRFS_I(inode)->defrag_compress)
return 1;
/* bad compression ratios */
if (BTRFS_I(inode)->flags & BTRFS_INODE_NOCOMPRESS)
return 0;
/* force compress */
if (btrfs_test_opt(fs_info, FORCE_COMPRESS))
return 1;
if (btrfs_test_opt(fs_info, COMPRESS) ||
BTRFS_I(inode)->flags & BTRFS_INODE_COMPRESS ||
BTRFS_I(inode)->prop_compress)
return btrfs_compress_heuristic(inode, start, end);
return 0;
}
>
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