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

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