linux-kernel - Re: [PATCH V2] f2fs: fix to adjust appropriate length for fiemap

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Message-ID: <9199e9fc-7b5b-4069-b79b-65ba5ae1b0f6@kernel.org>
Date: Wed, 6 Nov 2024 10:16:20 +0800
From: Chao Yu <chao@...nel.org>
To: Zhiguo Niu <niuzhiguo84@...il.com>
Cc: Chao Yu <chao@...nel.org>, Zhiguo Niu <zhiguo.niu@...soc.com>,
 jaegeuk@...nel.org, linux-f2fs-devel@...ts.sourceforge.net,
 linux-kernel@...r.kernel.org, ke.wang@...soc.com, Hao_hao.Wang@...soc.com
Subject: Re: [PATCH V2] f2fs: fix to adjust appropriate length for fiemap

On 2024/11/5 19:02, Zhiguo Niu wrote:
> Chao Yu <chao@...nel.org> 于2024年11月5日周二 18:39写道：
>>
>> On 2024/11/5 15:28, Zhiguo Niu wrote:
>>> Chao Yu <chao@...nel.org> 于2024年11月5日周二 15:04写道：
>>>>
>>>> On 2024/11/4 9:56, Zhiguo Niu wrote:
>>>>> If user give a file size as "length" parameter for fiemap
>>>>> operations, but if this size is non-block size aligned,
>>>>> it will show 2 segments fiemap results even this whole file
>>>>> is contiguous on disk, such as the following results:
>>>>>
>>>>>     ./f2fs_io fiemap 0 19034 ylog/analyzer.py
>>>>> Fiemap: offset = 0 len = 19034
>>>>>            logical addr.    physical addr.   length           flags
>>>>> 0       0000000000000000 0000000020baa000 0000000000004000 00001000
>>>>> 1       0000000000004000 0000000020bae000 0000000000001000 00001001
>>>>>
>>>>> after this patch:
>>>>> ./f2fs_io fiemap 0 19034 ylog/analyzer.py
>>>>> Fiemap: offset = 0 len = 19034
>>>>>        logical addr.    physical addr.   length           flags
>>>>> 0    0000000000000000 00000000315f3000 0000000000005000 00001001
>>>>>
>>>>> Signed-off-by: Zhiguo Niu <zhiguo.niu@...soc.com>
>>>>> ---
>>>>> V2: correct commit msg according to Chao's questions
>>>>> f2fs_io has been modified for testing, the length for fiemap is
>>>>> real file size, not block number
>>>>> ---
>>>>>     fs/f2fs/data.c | 4 ++--
>>>>>     1 file changed, 2 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>>>>> index 306b86b0..9fc229d 100644
>>>>> --- a/fs/f2fs/data.c
>>>>> +++ b/fs/f2fs/data.c
>>>>> @@ -1966,8 +1966,8 @@ int f2fs_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
>>>>>                         goto out;
>>>>>         }
>>>>>
>>>>> -     if (bytes_to_blks(inode, len) == 0)
>>>>> -             len = blks_to_bytes(inode, 1);
>>>>> +     if (len & (blks_to_bytes(inode, 1) - 1))
>>>>> +             len = round_up(len, blks_to_bytes(inode, 1));
>>>>
>>>> How do you think of getting rid of above alignment for len?
>>>>
>>>>>
>>>>>         start_blk = bytes_to_blks(inode, start);
>>>>>         last_blk = bytes_to_blks(inode, start + len - 1);
>>>>
>>>> And round up end position w/:
>>>>
>>>> last_blk = bytes_to_blks(inode, round_up(start + len - 1, F2FS_BLKSIZE));
>>> Hi Chao,
>>> I think this will change the current code logic
>>> -------------
>>> if (start_blk > last_blk)
>>>       goto out;
>>> -------------
>>> for example, a file with size 19006, but the length from the user is 16384.
>>> before this modification,  last_blk =  bytes_to_blks(inode, start +
>>> len - 1) = (inode, 16383) = 3
>>> after the first f2fs_map_blocks(). start_blk change to be 4,
>>> after the second f2fs_map_blocks(), fiemap_fill_nex_exten will be
>>> called to fill user parameter and then
>>> will goto out because start_blk > last_blk, then fiemap flow finishes.
>>> but after this modification, last_blk will be 4
>>> will do f2fs_map_blocks() until reach the max_file_blocks(inode)
>>
>> Yes, you're right, however, w/ this patch, it may change last_blk, e.g.
>>
>> xfs_io file -c "fiemap -v 0 19006" vs xfs_io file -c "fiemap -v 2 19006"
>> start_blk and last_blk will be: 0, 4 and 0, 5.
> Hi Chao,
> yes, but w/o this patch , the original code still has the same situation??
> for example
> xfs_io file -c "fiemap -v 0 16384" vs xfs_io file -c "fiemap -v 2 16384"
> start_blk and last_blk will be: 0, 3 and 0, 4.

For the case "fiemap -v 2 19006", offset is 2, and length is 19006, so last_offset
is 19008, and last_blk should be 4 rather than 5, right?

And for you case, it calculates last_blk correctly.

Thanks,

> but overall last_blk will change loop counts but has not affect on the results.
>>
>> Should we round_up len after start_blk & last_blk calculation?
> I thinks it is ok ,but just a little bit redundant with the following
> handling about len.
> 
> if (bytes_to_blks(inode, len) == 0)
>     len = blks_to_bytes(inode, 1);
> 
> Based on the above situation,
> do you have any other good suggestions? ^^
> thanks!
> 
>>
>> Thanks,
>>
>>> thanks！
>>>>
>>>> Thanks,
>>>>
>>