linux-kernel - Re: [PATCH v3] f2fs: avoid splitting bio when reading multiple pages

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Message-ID: <d2ac0da9-3d47-4269-a7b0-a18719c64346@kernel.org>
Date: Wed, 25 Jun 2025 16:45:06 +0800
From: Chao Yu <chao@...nel.org>
To: Jianan Huang <huangjianan@...omi.com>,
 linux-f2fs-devel@...ts.sourceforge.net, jaegeuk@...nel.org
Cc: chao@...nel.org, wanghui33@...omi.com, linux-kernel@...r.kernel.org,
 Sheng Yong <shengyong1@...omi.com>
Subject: Re: [PATCH v3] f2fs: avoid splitting bio when reading multiple pages

On 6/25/25 14:49, Jianan Huang wrote:
> When fewer pages are read, nr_pages may be smaller than nr_cpages. Due
> to the nr_vecs limit, the compressed pages will be split into multiple
> bios and then merged at the block level. In this case, nr_cpages should
> be used to pre-allocate bvecs.
> To handle this case, align max_nr_pages to cluster_size, which should be
> enough for all compressed pages.
> 
> Signed-off-by: Jianan Huang <huangjianan@...omi.com>
> Signed-off-by: Sheng Yong <shengyong1@...omi.com>
> ---
> Changes since v2:
> - Initialize index only for compressed files.
> Changes since v1:
> - Use aligned nr_pages instead of nr_cpages to pre-allocate bvecs.
> 
>  fs/f2fs/data.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> index 31e892842625..d071d9f6a811 100644
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -2303,7 +2303,7 @@ int f2fs_read_multi_pages(struct compress_ctx *cc, struct bio **bio_ret,
>  		}
>  
>  		if (!bio) {
> -			bio = f2fs_grab_read_bio(inode, blkaddr, nr_pages,
> +			bio = f2fs_grab_read_bio(inode, blkaddr, nr_pages - i,

Jianan,

Another case:

read page #0,1,2,3 from block #1000,1001,1002, cluster_size=4.

nr_pages=4
max_nr_pages=round_up(0+4,4)-round_down(0,4)=4

f2fs_mpage_readpages() calls f2fs_read_multi_pages() when nr_pages=1, at
that time, max_nr_pages equals to 1 as well.

f2fs_grab_read_bio(..., 1 - 0,...) allocate bio w/ 1 vec capacity, however,
we need at least 3 vecs to merge all cpages, right?

Thanks,

>  					f2fs_ra_op_flags(rac),
>  					folio->index, for_write);
>  			if (IS_ERR(bio)) {
> @@ -2376,6 +2376,14 @@ static int f2fs_mpage_readpages(struct inode *inode,
>  	unsigned max_nr_pages = nr_pages;
>  	int ret = 0;
>  
> +#ifdef CONFIG_F2FS_FS_COMPRESSION
> +	if (f2fs_compressed_file(inode)) {
> +		index = rac ? readahead_index(rac) : folio->index;
> +		max_nr_pages = round_up(index + nr_pages, cc.cluster_size) -
> +				round_down(index, cc.cluster_size);
> +	}
> +#endif
> +
>  	map.m_pblk = 0;
>  	map.m_lblk = 0;
>  	map.m_len = 0;
> @@ -2385,7 +2393,7 @@ static int f2fs_mpage_readpages(struct inode *inode,
>  	map.m_seg_type = NO_CHECK_TYPE;
>  	map.m_may_create = false;
>  
> -	for (; nr_pages; nr_pages--) {
> +	for (; nr_pages; nr_pages--, max_nr_pages--) {
>  		if (rac) {
>  			folio = readahead_folio(rac);
>  			prefetchw(&folio->flags);