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

[Chao Yu <chao@kernel.org>]

On 2021/12/15 2:56, Jaegeuk Kim wrote:
> On 12/12, Chao Yu wrote:
>> 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...
> 
> How about checking i_blocks and i_size instead of checking the entire map?

How about v2?

Thanks,

> 
>>>
>>> Thanks,
>>>
>>>>
>>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>>>
>>>>>>>> +
>>>>>>>>             err = f2fs_preallocate_blocks(iocb, from);
>>>>>>>>             if (err) {
>>>>>>>>     out_err:
>>>>>>>> -- 
>>>>>>>> 2.32.0
>>>>>>
>>>>>>
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