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

[Nathan Chancellor <natechancellor@gmail.com>]

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;
      |     ^~~~
...

Cheers,
Nathan