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 18:57:41 +0100
From:   Anders Blomdell <anders.blomdell@...trol.lth.se>
To:     Chuck Lever III <chuck.lever@...cle.com>,
        Benjamin Coddington <bcodding@...hat.com>
CC:     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 2022-11-23 18:51, Chuck Lever III wrote:
> 
>> 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.
My guess is that one has to look very hard at qcow2 handling in qemu...

/Anders


-- 
Anders Blomdell                  Email: anders.blomdell@...trol.lth.se
Department of Automatic Control
Lund University                  Phone:    +46 46 222 4625
P.O. Box 118
SE-221 00 Lund, Sweden