linux-kernel - Re: [PATCH] squashfs: provides backing_dev

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Date:   Tue, 16 Nov 2021 20:50:32 +0000
From:   Phillip Lougher <phillip@...ashfs.org.uk>
To:     Zheng Liang <zhengliang6@...wei.com>, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     yi.zhang@...wei.com, houtao1@...wei.com, miaoxie@...wei.com
Subject: Re: [PATCH] squashfs: provides backing_dev_info in order to disable
 read-ahead

On 16/11/2021 11:31, Zheng Liang wrote:
> the commit c1f6925e1091("mm: put readahead pages in cache earlier")
> causes the read performance of squashfs to deteriorate.Through testing,
> we find that the performance will be back by closing the readahead of
> squashfs. So we want to learn the way of ubifs, provides backing_dev_info
> and disable read-ahead.
> 
> --------------------------------------------------------------------
> We tested the following data by fio.
> squashfs image blocksize=128K
> test command:
> fio --name basic --bs=? --filename="/mnt/test_file" --rw=? --iodepth=1 --ioengine=psync --runtime=200 --time_based
> 
> turn on squashfs readahead in 5.10 kernel
> bs(k)      read/randread           MB/s
> 4            randread              271
> 128          randread              231
> 1024         randread              246
> 4            read                  310
> 128          read                  245
> 1024         read                  247
> 
> turn off squashfs readahead in 5.10 kernel
> bs(k)      read/randread           MB/s
> 4            randread              293
> 128          randread              330
> 1024         randread              363
> 4            read                  338
> 128          read                  360
> 1024         read                  365
> 
> turn on squashfs readahead and revert the
> commit c1f6925e1091("mm: put readahead
> pages in cache earlier") in 5.10 kernel
> bs(k)      read/randread           MB/s
> 4           randread               289
> 128         randread               306
> 1024        randread               335
> 4           read                   337
> 128         read                   336
> 1024        read                   338
> 
> Signed-off-by: Zheng Liang <zhengliang6@...wei.com>

Signed-off-by: Phillip Lougher <phillip@...ashfs.org.uk>

Looks good to me.

Phillip
--
Squashfs maintainer.

> ---
>   fs/squashfs/super.c | 33 +++++++++++++++++++++++++++++++++
>   1 file changed, 33 insertions(+)
> 
> diff --git a/fs/squashfs/super.c b/fs/squashfs/super.c
> index bb44ff4c5cc6..7a4c865fc8e6 100644
> --- a/fs/squashfs/super.c
> +++ b/fs/squashfs/super.c
> @@ -29,6 +29,7 @@
>   #include <linux/module.h>
>   #include <linux/magic.h>
>   #include <linux/xattr.h>
> +#include <linux/backing-dev.h>
>   
>   #include "squashfs_fs.h"
>   #include "squashfs_fs_sb.h"
> @@ -112,6 +113,24 @@ static const struct squashfs_decompressor *supported_squashfs_filesystem(
>   	return decompressor;
>   }
>   
> +static int squashfs_bdi_init(struct super_block *sb)
> +{
> +	int err;
> +	unsigned int major = MAJOR(sb->s_dev);
> +	unsigned int minor = MINOR(sb->s_dev);
> +
> +	bdi_put(sb->s_bdi);
> +	sb->s_bdi = &noop_backing_dev_info;
> +
> +	err = super_setup_bdi_name(sb, "squashfs_%u_%u", major, minor);
> +	if (err)
> +		return err;
> +
> +	sb->s_bdi->ra_pages = 0;
> +	sb->s_bdi->io_pages = 0;
> +
> +	return 0;
> +}
>   
>   static int squashfs_fill_super(struct super_block *sb, struct fs_context *fc)
>   {
> @@ -127,6 +146,20 @@ static int squashfs_fill_super(struct super_block *sb, struct fs_context *fc)
>   
>   	TRACE("Entered squashfs_fill_superblock\n");
>   
> +	/*
> +	 * squashfs provides 'backing_dev_info' in order to disable read-ahead. For
> +	 * squashfs, I/O is not deferred, it is done immediately in readpage,
> +	 * which means the user would always have to wait their own I/O. So the effect
> +	 * of readahead is very weak for squashfs. squashfs_bdi_init will set
> +	 * sb->s_bdi->ra_pages and sb->s_bdi->io_pages to 0 and close readahead for
> +	 * squashfs.
> +	 */
> +	err = squashfs_bdi_init(sb);
> +	if (err) {
> +		errorf(fc, "squashfs init bdi failed");
> +		return err;
> +	}
> +
>   	sb->s_fs_info = kzalloc(sizeof(*msblk), GFP_KERNEL);
>   	if (sb->s_fs_info == NULL) {
>   		ERROR("Failed to allocate squashfs_sb_info\n");
>