Re: [f2fs-dev] [PATCH] f2fs: skip f2fs_preallocate_blocks() for overwrite case

[Chao Yu <chao@kernel.org>]

Ping,

On 2021/10/30 11:02, Chao Yu wrote:
> On 2021/10/30 1:43, Jaegeuk Kim wrote:
>> On 10/29, Chao Yu wrote:
>>> Ping,
>>>
>>> On 2021/9/29 8:05, Chao Yu wrote:
>>>> On 2021/9/29 3:08, Jaegeuk Kim wrote:
>>>>> On 09/28, Chao Yu wrote:
>>>>>> In f2fs_file_write_iter(), let's use f2fs_overwrite_io() to
>>>>>> check whethere it is overwrite case, for such case, we can skip
>>>>>> f2fs_preallocate_blocks() in order to avoid f2fs_do_map_lock(),
>>>>>> which may be blocked by checkpoint() potentially.
>>>>>>
>>>>>> Signed-off-by: Chao Yu <chao@kernel.org>
>>>>>> ---
>>>>>>    fs/f2fs/file.c | 4 ++++
>>>>>>    1 file changed, 4 insertions(+)
>>>>>>
>>>>>> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
>>>>>> index 13deae03df06..51fecb2f4db5 100644
>>>>>> --- a/fs/f2fs/file.c
>>>>>> +++ b/fs/f2fs/file.c
>>>>>> @@ -4321,6 +4321,10 @@ static ssize_t f2fs_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
>>>>>>            preallocated = true;
>>>>>>            target_size = iocb->ki_pos + iov_iter_count(from);
>>>>>> +        if (f2fs_overwrite_io(inode, iocb->ki_pos,
>>>>>> +                        iov_iter_count(from)))
>>>>>> +            goto write;
>>>>>
>>>>> This calls f2fs_map_blocks() which can be duplicate, if it's not the overwirte
>>>>> case. Do we have other benefit?
>>>>
>>>> f2fs_overwrite_io() will break for append write case w/ below check:
>>>>
>>>>       if (pos + len > i_size_read(inode))
>>>>           return false;
>>>>
>>>> I guess we may only suffer double f2fs_map_blocks() for write hole
>>>> case, e.g. truncate to large size & write inside the filesize. For
>>>> this case, how about adding a condition to allow double f2fs_map_blocks()
>>>> only if write size is smaller than a threshold?
>>
>> I still don't see the benefit much to do double f2fs_map_blocks. What is the
>> problem here?
> 
> There is potential hangtask happened during swapfile's writeback:
> 
> - loop_kthread_worker_fn
>   - kthread_worker_fn
>    - loop_queue_work
>     - lo_rw_aio
>      - f2fs_file_write_iter
>       - f2fs_preallocate_blocks
>        - f2fs_map_blocks
>         - down_read
>          - rwsem_down_read_slowpath
>           - schedule
> 
> I try to mitigate such issue by preallocating swapfile's block address and
> avoid f2fs_do_map_lock() as much as possible in swapfile's writeback path...
> 
> Thanks,
> 
>>
>>>>
>>>> Thanks,
>>>>
>>>>>
>>>>>> +
>>>>>>            err = f2fs_preallocate_blocks(iocb, from);
>>>>>>            if (err) {
>>>>>>    out_err:
>>>>>> -- 
>>>>>> 2.32.0
>>>>
>>>>
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