linux-kernel - Re: [PATCH v2] nvdimm: virtio_pmem: serialize flush requests

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Message-ID: <69825d866edd7_44a2210014@iweiny-mobl.notmuch>
Date: Tue, 3 Feb 2026 14:41:42 -0600
From: Ira Weiny <ira.weiny@...el.com>
To: "Michael S. Tsirkin" <mst@...hat.com>, Li Chen <me@...ux.beauty>
CC: Pankaj Gupta <pankaj.gupta.linux@...il.com>, Dan Williams
	<dan.j.williams@...el.com>, Vishal Verma <vishal.l.verma@...el.com>, "Dave
 Jiang" <dave.jiang@...el.com>, Ira Weiny <ira.weiny@...el.com>, Cornelia Huck
	<cohuck@...hat.com>, Yuval Shaia <yuval.shaia@...cle.com>,
	<virtualization@...ts.linux.dev>, <nvdimm@...ts.linux.dev>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] nvdimm: virtio_pmem: serialize flush requests

Michael S. Tsirkin wrote:
> On Tue, Feb 03, 2026 at 10:13:51AM +0800, Li Chen wrote:
> > Under heavy concurrent flush traffic, virtio-pmem can overflow its request
> > virtqueue (req_vq): virtqueue_add_sgs() starts returning -ENOSPC and the
> > driver logs "no free slots in the virtqueue". Shortly after that the
> > device enters VIRTIO_CONFIG_S_NEEDS_RESET and flush requests fail with
> > "virtio pmem device needs a reset".
> > 
> > Serialize virtio_pmem_flush() with a per-device mutex so only one flush
> > request is in-flight at a time. This prevents req_vq descriptor overflow
> > under high concurrency.
> > 
> > Reproducer (guest with virtio-pmem):
> >   - mkfs.ext4 -F /dev/pmem0
> >   - mount -t ext4 -o dax,noatime /dev/pmem0 /mnt/bench
> >   - fio: ioengine=io_uring rw=randwrite bs=4k iodepth=64 numjobs=64
> >         direct=1 fsync=1 runtime=30s time_based=1
> >   - dmesg: "no free slots in the virtqueue"
> >            "virtio pmem device needs a reset"
> > 
> > Fixes: 6e84200c0a29 ("virtio-pmem: Add virtio pmem driver")
> > Signed-off-by: Li Chen <me@...ux.beauty>
> 
> 
> Thanks!
> And the commit message looks good now and includes the
> reproducer.
> 
> Acked-by: Michael S. Tsirkin <mst@...hat.com>
> 
> Ira are you picking this up?
> 

Yes queued for 7.0

Ira

[snip]