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Subject: Re: [PATCH] f2fs: skip f2fs_preallocate_blocks() for overwrite case
To:     Jaegeuk Kim <jaegeuk@kernel.org>
Cc:     linux-f2fs-devel@lists.sourceforge.net,
        linux-kernel@vger.kernel.org
References: <20210928151911.11189-1-chao@kernel.org>
 <YVNoHudG5c65X85G@google.com>
From:   Chao Yu <chao@kernel.org>
Message-ID: <65f6c366-9e5b-fe7f-7c38-061996d1882b@kernel.org>
Date:   Wed, 29 Sep 2021 08:05:12 +0800
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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?

Thanks,

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