On 2018年05月15日 16:21, Nikolay Borisov wrote:
>
>
> 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 :
In that case, you won't go into compress_file_range() at all.
As the only caller of compress_file_range() is async_cow_start(), which
get queued in cow_file_range_async().
And cow_file_range_async() traces back to run_delalloc_range().
Here we determine (basically) where some dirty range goes.
The modification in inode_need_compress() mostly affects the decision in
run_delalloc_range(), so we won't go cow_file_range_async(), thus we
won't hit the problem you described.
>
> 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;
>
Not exactly.
Force-compress should less us bypass bad compression ratio, so it should
be at least before ratio check.
Thanks,
Qu
> /* 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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