netdev - Re: [PATCH net-next v3 3/3] page_pool: fix IOMMU crash when driver has already unbound

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Message-ID: <87o731by64.fsf@toke.dk>
Date: Wed, 30 Oct 2024 12:57:55 +0100
From: Toke Høiland-Jørgensen <toke@...hat.com>
To: Yunsheng Lin <linyunsheng@...wei.com>, Jesper Dangaard Brouer
 <hawk@...nel.org>, davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com
Cc: zhangkun09@...wei.com, fanghaiqing@...wei.com, liuyonglong@...wei.com,
 Robin Murphy <robin.murphy@....com>, Alexander Duyck
 <alexander.duyck@...il.com>, IOMMU <iommu@...ts.linux.dev>, Andrew Morton
 <akpm@...ux-foundation.org>, Eric Dumazet <edumazet@...gle.com>, Ilias
 Apalodimas <ilias.apalodimas@...aro.org>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, netdev@...r.kernel.org, kernel-team
 <kernel-team@...udflare.com>
Subject: Re: [PATCH net-next v3 3/3] page_pool: fix IOMMU crash when driver
 has already unbound

Yunsheng Lin <linyunsheng@...wei.com> writes:

>> But, well, I'm not sure it is? You seem to be taking it as axiomatic
>> that the wait in itself is bad. Why? It's just a bit memory being held
>> on to while it is still in use, and so what?
>
> Actually, I thought about adding some sort of timeout or kicking based on
> jakub's waiting patch too.
>
> But after looking at more caching in the networking, waiting and kicking/flushing
> seems harder than recording the inflight pages, mainly because kicking/flushing
> need very subsystem using page_pool owned page to provide a kicking/flushing
> mechanism for it to work, not to mention how much time does it take to do all
> the kicking/flushing.

Eliding the details above, but yeah, you're right, there are probably
some pernicious details to get right if we want to flush all caches. S
I wouldn't do that to start with. Instead, just add the waiting to start
with, then wait and see if this actually turns out to be a problem in
practice. And if it is, identify the source of that problem, deal with
it, rinse and repeat :)

-Toke