On 2018年05月15日 16:35, Nikolay Borisov wrote:
>
>
> 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?id9707)
>>>>
>>>> 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 =trfs_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?
Yep, that's true.
> Perhaps
> will_compress can also be removed etc? As it stands currently it's
> spaghetti code.
Nice idea to further clean this code up.
I'll update both patch after receiving enough feedback.
Thanks,
Qu
>
>>>
>>> ret =ow_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 =ow_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 =trfs_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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