On 15.05.2018 11:30, Qu Wenruo wrote:
>
>
> 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.
So you have re-iterated what I've described further below. This means it
should be possible to remove the invocation of inode_need_compress in
compress_file_range and simplify the code there, no? Perhaps
will_compress can also be removed etc? As it stands currently it's
spaghetti code.
>>
>> 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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