linux-kernel - Re: [PATCH v2 1/6] mm/gup: Provide gup_get

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Date:   Thu, 26 Nov 2020 14:02:02 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     kan.liang@...ux.intel.com, mingo@...nel.org, acme@...nel.org,
        mark.rutland@....com, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, eranian@...gle.com, christophe.leroy@...roup.eu,
        npiggin@...il.com, linuxppc-dev@...ts.ozlabs.org,
        mpe@...erman.id.au, will@...nel.org, aneesh.kumar@...ux.ibm.com,
        sparclinux@...r.kernel.org, davem@...emloft.net,
        catalin.marinas@....com, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, ak@...ux.intel.com,
        dave.hansen@...el.com, kirill.shutemov@...ux.intel.com
Subject: Re: [PATCH v2 1/6] mm/gup: Provide gup_get_pte() more generic

On Thu, Nov 26, 2020 at 12:43:00PM +0000, Matthew Wilcox wrote:
> On Thu, Nov 26, 2020 at 01:01:15PM +0100, Peter Zijlstra wrote:
> > +#ifdef CONFIG_GUP_GET_PTE_LOW_HIGH
> > +/*
> > + * WARNING: only to be used in the get_user_pages_fast() implementation.
> > + * With get_user_pages_fast(), we walk down the pagetables without taking any
> > + * locks.  For this we would like to load the pointers atomically, but sometimes
> > + * that is not possible (e.g. without expensive cmpxchg8b on x86_32 PAE).  What
> > + * we do have is the guarantee that a PTE will only either go from not present
> > + * to present, or present to not present or both -- it will not switch to a
> > + * completely different present page without a TLB flush in between; something
> > + * that we are blocking by holding interrupts off.
> 
> I feel like this comment needs some love.  How about:
> 
>  * For walking the pagetables without holding any locks.  Some architectures
>  * (eg x86-32 PAE) cannot load the entries atomically without using
>  * expensive instructions.  We are guaranteed that a PTE will only either go
>  * from not present to present, or present to not present -- it will not
>  * switch to a completely different present page without a TLB flush
>  * inbetween; which we are blocking by holding interrupts off.
> 
> And it would be nice to have an assertion that interrupts are disabled
> in the code.  Because comments are nice, but nobody reads them.

Quite agreed, I'll stick a separate patch on with the updated comment
and a lockdep_assert_irqs_disabled() in. I'm afraid that latter will make
for header soup though :/

We'll see, let the robots have it.