netdev - Re: [PATCH net v2 2/2] page_pool: fix IOMMU crash when driver has already unbound

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Message-ID: <CAC_iWjLBE9UY2wk_kKE=t=npRBF13HoLWODLUpQJ6F3P8sv4rw@mail.gmail.com>
Date: Wed, 2 Oct 2024 11:23:22 +0300
From: Ilias Apalodimas <ilias.apalodimas@...aro.org>
To: Paolo Abeni <pabeni@...hat.com>
Cc: Yunsheng Lin <yunshenglin0825@...il.com>, Yunsheng Lin <linyunsheng@...wei.com>, 
	liuyonglong@...wei.com, fanghaiqing@...wei.com, zhangkun09@...wei.com, 
	Robin Murphy <robin.murphy@....com>, Alexander Duyck <alexander.duyck@...il.com>, 
	IOMMU <iommu@...ts.linux.dev>, Wei Fang <wei.fang@....com>, 
	Shenwei Wang <shenwei.wang@....com>, Clark Wang <xiaoning.wang@....com>, 
	Eric Dumazet <edumazet@...gle.com>, Tony Nguyen <anthony.l.nguyen@...el.com>, 
	Przemek Kitszel <przemyslaw.kitszel@...el.com>, 
	Alexander Lobakin <aleksander.lobakin@...el.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>, 
	John Fastabend <john.fastabend@...il.com>, Saeed Mahameed <saeedm@...dia.com>, 
	Leon Romanovsky <leon@...nel.org>, Tariq Toukan <tariqt@...dia.com>, Felix Fietkau <nbd@....name>, 
	Lorenzo Bianconi <lorenzo@...nel.org>, Ryder Lee <ryder.lee@...iatek.com>, 
	Shayne Chen <shayne.chen@...iatek.com>, Sean Wang <sean.wang@...iatek.com>, 
	Kalle Valo <kvalo@...nel.org>, Matthias Brugger <matthias.bgg@...il.com>, 
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, imx@...ts.linux.dev, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, intel-wired-lan@...ts.osuosl.org, 
	bpf@...r.kernel.org, linux-rdma@...r.kernel.org, 
	linux-wireless@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	linux-mediatek@...ts.infradead.org, linux-mm@...ck.org, davem@...emloft.net, 
	kuba@...nel.org
Subject: Re: [PATCH net v2 2/2] page_pool: fix IOMMU crash when driver has
 already unbound

Hi Paolo,

On Wed, 2 Oct 2024 at 10:38, Paolo Abeni <pabeni@...hat.com> wrote:
>
> Hi,
>
> On 10/2/24 04:34, Yunsheng Lin wrote:
> > On 10/1/2024 9:32 PM, Paolo Abeni wrote:
> >> Is the problem only tied to VFs drivers? It's a pity all the page_pool
> >> users will have to pay a bill for it...
> >
> > I am afraid it is not only tied to VFs drivers, as:
> > attempting DMA unmaps after the driver has already unbound may leak
> > resources or at worst corrupt memory.
> >
> > Unloading PFs driver might cause the above problems too, I guess the
> > probability of crashing is low for the PF as PF can not be disable
> > unless it can be hot-unplug'ed, but the probability of leaking resources
> > behind the dma mapping might be similar.
>
> Out of sheer ignorance, why/how the refcount acquired by the page pool
> on the device does not prevent unloading?
>
> I fear the performance impact could be very high: AFICS, if the item
> array become fragmented, insertion will take linar time, with the quite
> large item_count/pool size. If so, it looks like a no-go.

It would be could if someone could test that. I'll look around in case
we have any test machines with cards that run on page pool.

>
> I fear we should consider blocking the device removal until all the
> pages are returned/unmapped ?!? (I hope that could be easier/faster)

Jakub send an RFC doing that [0]. Yes, this is far far simpler and
does not affect performance, but aren't we implicitly breaking
userspace?

[0] https://lore.kernel.org/netdev/20240806151618.1373008-1-kuba@kernel.org/

Thanks
/Ilias
>
> /P
>