linux-kernel - Re: [PATCH] nfs: set block size according to pnfs

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Date:   Wed, 16 Jun 2021 13:47:13 +0000
From:   Trond Myklebust <trondmy@...merspace.com>
To:     "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
        "hsiangkao@...ux.alibaba.com" <hsiangkao@...ux.alibaba.com>
CC:     "joseph.qi@...ux.alibaba.com" <joseph.qi@...ux.alibaba.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "anna.schumaker@...app.com" <anna.schumaker@...app.com>
Subject: Re: [PATCH] nfs: set block size according to pnfs_blksize first

On Wed, 2021-06-16 at 20:44 +0800, Gao Xiang wrote:
> When testing fstests with ext4 over nfs 4.2, I found generic/486
> failed. The root cause is that the length of its xattr value is
>   min(st_blksize * 3 / 4, XATTR_SIZE_MAX)
> 
> which is 4096 * 3 / 4 = 3072 for underlayfs ext4 rather than
> XATTR_SIZE_MAX = 65536 for nfs since the block size would be wsize
> (=131072) if bsize is not specified.
> 
> Let's use pnfs_blksize first instead of using wsize directly if
> bsize isn't specified. And the testcase itself can pass now.
> 
> Cc: Trond Myklebust <trond.myklebust@...merspace.com>
> Cc: Anna Schumaker <anna.schumaker@...app.com>
> Cc: Joseph Qi <joseph.qi@...ux.alibaba.com>
> Signed-off-by: Gao Xiang <hsiangkao@...ux.alibaba.com>
> ---
> Considering bsize is not specified, we might use pnfs_blksize
> directly first rather than wsize.
> 
>  fs/nfs/super.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/nfs/super.c b/fs/nfs/super.c
> index fe58525cfed4..5015edf0cd9a 100644
> --- a/fs/nfs/super.c
> +++ b/fs/nfs/super.c
> @@ -1068,9 +1068,13 @@ static void nfs_fill_super(struct super_block
> *sb, struct nfs_fs_context *ctx)
>         snprintf(sb->s_id, sizeof(sb->s_id),
>                  "%u:%u", MAJOR(sb->s_dev), MINOR(sb->s_dev));
>  
> -       if (sb->s_blocksize == 0)
> -               sb->s_blocksize = nfs_block_bits(server->wsize,
> +       if (sb->s_blocksize == 0) {
> +               unsigned int blksize = server->pnfs_blksize ?
> +                       server->pnfs_blksize : server->wsize;

NACK. The pnfs block size is a layout driver-specific quantity, and
should not be used to substitute for the server-advertised block size.
It only applies to I/O _if_ the client is holding a layout for a
specific file and is using pNFS to do I/O to that file.

It has nothing to do with xattrs at all.

> +
> +               sb->s_blocksize = nfs_block_bits(blksize,
>                                                  &sb-
> >s_blocksize_bits);
> +       }
>  
>         nfs_super_set_maxbytes(sb, server->maxfilesize);
>         server->has_sec_mnt_opts = ctx->has_sec_mnt_opts;

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@...merspace.com