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

From: Eric Biggers
Date: Mon Jun 29 2020 - 15:15:26 EST


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