linux-kernel - Re: [MOCKUP] x86/mm: Lightweight lazy mm refcounting

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Message-ID: <0239dde7da2d6b6499970f343c7498c711ce14c2.camel@surriel.com>
Date:   Thu, 03 Dec 2020 09:26:32 -0500
From:   Rik van Riel <riel@...riel.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Nicholas Piggin <npiggin@...il.com>,
        Anton Blanchard <anton@...abs.org>,
        Arnd Bergmann <arnd@...db.de>,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        X86 ML <x86@...nel.org>, Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [MOCKUP] x86/mm: Lightweight lazy mm refcounting

On Thu, 2020-12-03 at 12:31 +0000, Matthew Wilcox wrote:

> And this just makes me think RCU freeing of mm_struct.  I'm sure it's
> more complicated than that (then, or now), but if an anonymous
> process
> is borrowing a freed mm, and the mm is freed by RCU then it will not
> go
> away until the task context switches.  When we context switch back to
> the anon task, it'll borrow some other task's MM and won't even
> notice
> that the MM it was using has gone away.

One major complication here is that most of the
active_mm borrowing is done by the idle task,
but RCU does not wait for idle tasks to context
switch.

That means RCU, as it is today, is not a
mechanism that mm_struct freeing could just
piggyback off.

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