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SubjectRe: [f2fs-dev] [PATCH] f2fs: avoid readahead race condition
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On 2020/6/24 9:21, 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>
> ---
> fs/f2fs/data.c | 15 +++++++++++++++
> fs/f2fs/f2fs.h | 1 +
> fs/f2fs/super.c | 2 ++
> 3 files changed, 18 insertions(+)
>
> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> index dfd3225153570..1886d83bc5f15 100644
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -2292,6 +2292,7 @@ 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;
> @@ -2302,6 +2303,17 @@ 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 (F2FS_I(inode)->ra_offset == readahead_index(rac))
> + drop_ra = true;

I guess you missed to return at somewhere when drop_ra is true?

thanks,

> + else
> + F2FS_I(inode)->ra_offset = readahead_index(rac);
> + }
> +
> for (; nr_pages; nr_pages--) {
> if (rac) {
> page = readahead_page(rac);
> @@ -2368,6 +2380,9 @@ static int f2fs_mpage_readpages(struct inode *inode,
> }
> if (bio)
> __submit_bio(F2FS_I_SB(inode), bio, DATA);
> +
> + if (rac && readahead_count(rac) && !drop_ra)
> + F2FS_I(inode)->ra_offset = -1;
> return ret;
> }
>
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index 7fb2a1a334388..753782426feac 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -809,6 +809,7 @@ struct f2fs_inode_info {
> struct list_head inmem_pages; /* inmemory pages managed by f2fs */
> struct task_struct *inmem_task; /* store inmemory task */
> struct mutex inmem_lock; /* lock for inmemory pages */
> + pgoff_t ra_offset; /* ongoing readahead offset */
> struct extent_tree *extent_tree; /* cached extent_tree entry */
>
> /* avoid racing between foreground op and gc */
> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
> index 7326522057378..80cb7cd358f84 100644
> --- a/fs/f2fs/super.c
> +++ b/fs/f2fs/super.c
> @@ -1015,6 +1015,8 @@ static struct inode *f2fs_alloc_inode(struct super_block *sb)
> /* Will be used by directory only */
> fi->i_dir_level = F2FS_SB(sb)->dir_level;
>
> + fi->ra_offset = -1;
> +
> return &fi->vfs_inode;
> }
>
>

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