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Subject: Re: [f2fs-dev] [PATCH] f2fs: avoid readahead race condition
To:     Jaegeuk Kim <jaegeuk@kernel.org>, <linux-kernel@vger.kernel.org>,
        <linux-f2fs-devel@lists.sourceforge.net>, <kernel-team@android.com>
References: <20200624012148.180050-1-jaegeuk@kernel.org>
From:   Chao Yu <yuchao0@huawei.com>
Message-ID: <368cf853-0411-c237-f0ad-481f78a1eef3@huawei.com>
Date:   Sun, 28 Jun 2020 10:35:26 +0800
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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;
>  }
>  
>