linux-kernel - Commit 'sunrpc: Use sendmsg(MSG_SPLICE_PAGES) rather then sendpage' broke O

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Message-ID: <2d47431decaaf4bba0023c91ef0d7fd51b84333b.camel@redhat.com>
Date:   Thu, 17 Aug 2023 18:52:01 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     linux-nfs@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Commit 'sunrpc: Use sendmsg(MSG_SPLICE_PAGES) rather then sendpage'
 broke O_DIRECT over NFS

Hi!

I just updated my developement systems to 6.5-rc6 (from 6.4) and now I can't start a VM 
with a disk which is mounted over the NFS.

The VM has two qcow2 files, one depends on another and qemu opens both.

This is the command line of qemu:

-drive if=none,id=os_image,file=./disk_s1.qcow2,aio=native,discard=unmap,cache=none

The disk_s1.qcow2 depends on disk_s0.qcow2

However this is what I get:

qemu-system-x86_64: -drive if=none,id=os_image,file=./disk_s1.qcow2,aio=native,discard=unmap,cache=none: Could not open backing file: Could not open './QFI?': No such file or directory

'QFI?' is qcow2 file signature, which signals that there might be some nasty corruption happening.

The program was supposed to read a field inside the disk_s1.qcow2 file which should read 'disk_s0.qcow2' 
but instead it seems to read the first 4 bytes of the file.

Bisect leads to the above commit. Reverting it was not possible due to many changes.

Both the client and the server were tested with the 6.5-rc6 kernel, but once rebooting the server into
the 6.4, the bug disappeared, thus I did a bisect on the server.

When I tested a version before the offending commit on the server, the 6.5-rc6 client was able to work with it,
which increases the chances that the bug is in nfsd.

Switching qemu to use write back paging also helps (aio=threads,discard=unmap,cache=writeback)
The client and the server (both 6.5-rc6) work with this configuration.

Running the VM on the same machine (also 6.5-rc6) where the VM disk is located (thus avoiding NFS) works as well.

I tested several VMs that I have, all are affected in the same way.

I run somewhat outdated qemu, but running the latest qemu doesn't make a difference.

I use nfs4.

I can test patches and provide more info if needed.

Best regards,
	Maxim Levitsky