Re: [f2fs-dev] [PATCH v2] f2fs: avoid readahead race condition

[Eric Biggers]

On Mon, Jun 29, 2020 at 08:03:23AM -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@xxxxxxxxxx>
> ---
> 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, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> index d6094b9f3916..9b69a159cc6c 100644
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -2403,6 +2403,7 @@ int f2fs_mpage_readpages(struct address_space *mapping,
>  #endif
>  	unsigned max_nr_pages = nr_pages;
>  	int ret = 0;
> +	bool drop_ra = false;
>  
>  	map.m_pblk = 0;
>  	map.m_lblk = 0;
> @@ -2413,13 +2414,25 @@ int f2fs_mpage_readpages(struct address_space *mapping,
>  	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 (pages && is_readahead) {
> +		page = list_last_entry(pages, struct page, lru);
> +		if (F2FS_I(inode)->ra_offset == page_index(page))
> +			drop_ra = true;
> +		else
> +			F2FS_I(inode)->ra_offset = page_index(page);
> +	}

This is a data race because ra_offset can be read/written by different threads
concurrently.

It either needs locking, or READ_ONCE() and WRITE_ONCE() if races are okay.

- Eric