Re: [PATCH 1/2] btrfs: inode: Don't compress if NODATASUM or NODATACOW set

[Nikolay Borisov <nborisov@suse.com>]

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 <jamespharvey20@gmail.com>
>>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>>> ---
>>>  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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