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

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Message-ID: <YMoFcdhVwMXJQPJ+@B-P7TQMD6M-0146.local>
Date:   Wed, 16 Jun 2021 22:06:41 +0800
From:   Gao Xiang <hsiangkao@...ux.alibaba.com>
To:     Trond Myklebust <trondmy@...merspace.com>
Cc:     "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
        "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, Jun 16, 2021 at 01:47:13PM +0000, Trond Myklebust wrote:
> 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.

Honestly, I'm not sure if it's ok as well.

> 
> It has nothing to do with xattrs at all.

Yet my question is how to deal with generic/486, should we just skip
the case directly? I cannot find some proper way to get underlayfs
block size or real xattr value limit.

For now, generic/486 will return ENOSPC at
fsetxattr(fd, "user.world", value, 65536, XATTR_REPLACE);
when testing new nfs4.2 xattr support.

Thanks,
Gao Xiang

> 
> > +
> > +               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
> 
>