Re: [PATCH v2] btrfs: inode: Don't compress if NODATASUM or NODATACOW set

[WenRuo Qu <wqu@suse.com>]

On 2019/7/5 上午12:04, David Sterba wrote:
> On Mon, Jul 01, 2019 at 05:12:46AM +0000, 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 a 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, as compressed data could make the whole extent
>> unreadable, so there is no need to allow compression for
>> NODATACSUM.
>>
>> The fix will refactor the inode compression check into two parts:
>> - inode_can_compress()
>>   As the hard requirement, checked at btrfs_run_delalloc_range(), so no
>>   compression will happen for NODATASUM inode at all.
>>
>> - inode_need_compress()
>>   As the soft requirement, checked at btrfs_run_delalloc_range() and
>>   compress_file_range().
>>
>> Reported-by: James Harvey <jamespharvey20@gmail.com>
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>> ---
>> Changelog:
>> v2:
>> - Refactor inode_need_compress() into two functions
>> - Refactor inode_need_compress() to return bool
>> ---
>>  fs/btrfs/inode.c | 38 ++++++++++++++++++++++++++++++++------
>>  1 file changed, 32 insertions(+), 6 deletions(-)
>>
>> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
>> index a2aabdb85226..be1cabf35680 100644
>> --- a/fs/btrfs/inode.c
>> +++ b/fs/btrfs/inode.c
>> @@ -394,24 +394,49 @@ static noinline int add_async_extent(struct async_chunk *cow,
>>  	return 0;
>>  }
>>  
>> -static inline int inode_need_compress(struct inode *inode, u64 start, u64 end)
>> +/*
>> + * Check if the inode can accept compression.
>> + *
>> + * This checks for the hard requirement of compression, including CoW and
>> + * checksum requirement.
>> + */
>> +static inline bool inode_can_compress(struct inode *inode)
>> +{
>> +	if (BTRFS_I(inode)->flags & BTRFS_INODE_NODATACOW ||
>> +	    BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM)
>> +		return false;
>> +	return true;
>> +}
>> +
>> +/*
>> + * Check if the inode need compression.
>> + *
>> + * This checks for the soft requirement of compression.
>> + */
>> +static inline bool inode_need_compress(struct inode *inode, u64 start, u64 end)
>>  {
>>  	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
>>  
>> +	if (!inode_can_compress(inode)) {
>> +		WARN(IS_ENABLED(CONFIG_BTRFS_DEBUG),
>> +			KERN_ERR "BTRFS: unexpected compression for ino %llu\n",
>> +			btrfs_ino(BTRFS_I(inode)));
>> +		return false;
>> +	}
>>  	/* force compress */
>>  	if (btrfs_test_opt(fs_info, FORCE_COMPRESS))
>> -		return 1;
>> +		return true;
>>  	/* defrag ioctl */
>>  	if (BTRFS_I(inode)->defrag_compress)
>> -		return 1;
>> +		return true;
>>  	/* bad compression ratios */
>>  	if (BTRFS_I(inode)->flags & BTRFS_INODE_NOCOMPRESS)
>> -		return 0;
>> +		return false;
>>  	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;
>> +	return false;
>>  }
>>  
>>  static inline void inode_should_defrag(struct btrfs_inode *inode,
>> @@ -1630,7 +1655,8 @@ int btrfs_run_delalloc_range(struct inode *inode, struct page *locked_page,
>>  	} else if (BTRFS_I(inode)->flags & BTRFS_INODE_PREALLOC && !force_cow) {
>>  		ret = run_delalloc_nocow(inode, locked_page, start, end,
>>  					 page_started, 0, nr_written);
>> -	} else if (!inode_need_compress(inode, start, end)) {
>> +	} else if (!inode_can_compress(inode) ||
>> +		   !inode_need_compress(inode, start, end)) {
> 
> Well, that's not excatly what I expected, but because this is an
> important fix I won't object now and add it to 5.3 queue.
> 

I know what you expect, single inode_can_compress().

But still, we want to avoid hitting the compression routine, thus here
we do extra inode_need_compress() check other than exiting in
compress_file_extent().

Thanks,
Qu