linux-kernel - Re: [PATCH] nvme-tcp: fix usage of page_frag

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Message-ID: <20251001164152.GB4234@yadro.com>
Date: Wed, 1 Oct 2025 19:41:52 +0300
From: Dmitry Bogdanov <d.bogdanov@...ro.com>
To: Chris Leech <cleech@...hat.com>
CC: Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...nel.dk>, "Christoph
 Hellwig" <hch@....de>, Sagi Grimberg <sagi@...mberg.me>, Stuart Hayes
	<stuart.w.hayes@...il.com>, <linux-nvme@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>, <linux@...ro.com>, <stable@...r.kernel.org>
Subject: Re: [PATCH] nvme-tcp: fix usage of page_frag_cache

On Tue, Sep 30, 2025 at 11:31:26PM -0700, Chris Leech wrote:
> 
> On Mon, Sep 29, 2025 at 02:19:51PM +0300, Dmitry Bogdanov wrote:
> > nvme uses page_frag_cache to preallocate PDU for each preallocated request
> > of block device. Block devices are created in parallel threads,
> > consequently page_frag_cache is used in not thread-safe manner.
> > That leads to incorrect refcounting of backstore pages and premature free.
> >
> > That can be catched by !sendpage_ok inside network stack:
> >
> > WARNING: CPU: 7 PID: 467 at ../net/core/skbuff.c:6931 skb_splice_from_iter+0xfa/0x310.
> >       tcp_sendmsg_locked+0x782/0xce0
> >       tcp_sendmsg+0x27/0x40
> >       sock_sendmsg+0x8b/0xa0
> >       nvme_tcp_try_send_cmd_pdu+0x149/0x2a0
> > Then random panic may occur.
> >
> > Fix that by serializing the usage of page_frag_cache.
> 
> Thank you for reporting this. I think we can fix it without blocking the
> async namespace scanning with a mutex, by switching from a per-queue
> page_frag_cache to per-cpu. There shouldn't be a need to keep the
> page_frag allocations isolated by queue anyway.
> 
> It would be great if you could test the patch which I'll send after
> this.
> 

As I commented on your patch, a naive per-cpu cache solution is
error-prone. The complete solution will be unnecessaryly difficult.
Block device creation is not a data plane, it is a control plane, so
there is no sense to use there lockless algorithms.

My patch is a simple and error-proof already.
So, I insist on this solution.

BR,
 Dmitry