linux-kernel - Re: Kernel 6.0.5 breaks virtual machines that boot from nfs mounted qcow2 images

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Message-ID: <A0F05ED3-14B0-4AE7-B4F7-82398033CA34@redhat.com>
Date:   Wed, 23 Nov 2022 12:49:04 -0500
From:   Benjamin Coddington <bcodding@...hat.com>
To:     Anders Blomdell <anders.blomdell@...trol.lth.se>,
        Chuck Lever <chuck.lever@...cle.com>,
        Jeff Layton <jlayton@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>
Cc:     linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        Anna Schumaker <anna@...nel.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>
Subject: Re: Kernel 6.0.5 breaks virtual machines that boot from nfs mounted
 qcow2 images

On 23 Nov 2022, at 5:08, Anders Blomdell wrote:

> Our problems turned out to be a fallout of Al Viros's splice rework, where nfsd reads with non-zero offsets and not ending
> on a page boundary failed to remap the last page. I belive that this is a decent fix for that problem (tested on v6.1-rc6,
> 6.0.7 and 6.0.9)
>
> ---- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -873,7 +873,7 @@ nfsd_splice_actor(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
>         unsigned offset = buf->offset;
>          page += offset / PAGE_SIZE;
> -       for (int i = sd->len; i > 0; i -= PAGE_SIZE)
> +       for (int i = sd->len + offset % PAGE_SIZE; i > 0; i -= PAGE_SIZE)
>                 svc_rqst_replace_page(rqstp, page++);
>         if (rqstp->rq_res.page_len == 0)        // first call
>                 rqstp->rq_res.page_base = offset % PAGE_SIZE;


Does anyone have insight into how we could possibly have caught this in testing?

I have been unable to make my knfsd hit this problem: it returns unaligned READs all day long, no problems.

Ben