Re: [f2fs-dev] [PATCH v3] f2fs: avoid readahead race condition

[Chao Yu <yuchao0@huawei.com>]

On 2020/7/1 4:56, Jaegeuk Kim wrote:
> On 06/30, Nathan Chancellor wrote:
>> On Mon, Jun 29, 2020 at 01:27:20PM -0700, Jaegeuk Kim wrote:
>>> If two readahead threads having same offset enter in readpages, every read
>>> IOs are split and issued to the disk which giving lower bandwidth.
>>>
>>> This patch tries to avoid redundant readahead calls.
>>>
>>> Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
>>> ---
>>> v3:
>>>  - use READ|WRITE_ONCE
>>> v2:
>>>   - add missing code to bypass read
>>>  
>>>  fs/f2fs/data.c  | 18 ++++++++++++++++++
>>>  fs/f2fs/f2fs.h  |  1 +
>>>  fs/f2fs/super.c |  2 ++
>>>  3 files changed, 21 insertions(+)
>>>
>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>>> index 995cf78b23c5e..360b4c9080d97 100644
>>> --- a/fs/f2fs/data.c
>>> +++ b/fs/f2fs/data.c
>>> @@ -2296,6 +2296,7 @@ static int f2fs_mpage_readpages(struct inode *inode,
>>>  	unsigned nr_pages = rac ? readahead_count(rac) : 1;
>>>  	unsigned max_nr_pages = nr_pages;
>>>  	int ret = 0;
>>> +	bool drop_ra = false;
>>>  
>>>  	map.m_pblk = 0;
>>>  	map.m_lblk = 0;
>>> @@ -2306,10 +2307,24 @@ static int f2fs_mpage_readpages(struct inode *inode,
>>>  	map.m_seg_type = NO_CHECK_TYPE;
>>>  	map.m_may_create = false;
>>>  
>>> +	/*
>>> +	 * Two readahead threads for same address range can cause race condition
>>> +	 * which fragments sequential read IOs. So let's avoid each other.
>>> +	 */
>>> +	if (rac && readahead_count(rac)) {
>>> +		if (READ_ONCE(F2FS_I(inode)->ra_offset) == readahead_index(rac))
>>> +			drop_ra = true;
>>> +		else
>>> +			WRITE_ONCE(F2FS_I(inode)->ra_offset,
>>> +						readahead_index(rac));
>>> +	}
>>> +
>>>  	for (; nr_pages; nr_pages--) {
>>>  		if (rac) {
>>>  			page = readahead_page(rac);
>>>  			prefetchw(&page->flags);
>>> +			if (drop_ra)
>>> +				goto next_page;
>>
>> When CONFIG_F2FS_FS_COMPRESSION is not set (i.e. x86_64 defconfig +
>> CONFIG_F2FS_FS=y):
>>
>> $ make -skj"$(nproc)" O=out distclean defconfig fs/f2fs/data.o
>> ../fs/f2fs/data.c: In function ‘f2fs_mpage_readpages’:
>> ../fs/f2fs/data.c:2327:5: error: label ‘next_page’ used but not defined
>>  2327 |     goto next_page;
>>       |     ^~~~
>> ...
> 
> Thanks. I pushed the fix for -next.
> https://lore.kernel.org/linux-f2fs-devel/1be18397-7fc6-703e-121b-e210e101357f@infradead.org/T/#t

Reviewed-by: Chao Yu <yuchao0@huawei.com>

Thanks,

> 
> Thanks,
> 
>>
>> Cheers,
>> Nathan
>>
>>
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> 
> 
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