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

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Date:   Wed, 23 Nov 2022 17:51:37 +0000
From:   Chuck Lever III <chuck.lever@...cle.com>
To:     Benjamin Coddington <bcodding@...hat.com>
CC:     Anders Blomdell <anders.blomdell@...trol.lth.se>,
        Jeff Layton <jlayton@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        "linux-kernel@...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 Nov 23, 2022, at 12:49 PM, Benjamin Coddington <bcodding@...hat.com> wrote:
> 
> 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?

Was also wondering this. I had though fstests (via fsx) would have exercised
this usage scenario.


--
Chuck Lever