linux-kernel - Re: [PATCH] x86/mm: Fix flush_tlb_range() when used for zapping normal PMDs

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Message-ID: <20250122103347.GH7145@noisy.programming.kicks-ass.net>
Date: Wed, 22 Jan 2025 11:33:47 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Jann Horn <jannh@...gle.com>
Cc: Rik van Riel <riel@...riel.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Andy Lutomirski <luto@...nel.org>, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH] x86/mm: Fix flush_tlb_range() when used for zapping
 normal PMDs

On Fri, Jan 03, 2025 at 11:11:51PM +0100, Jann Horn wrote:
> On Fri, Jan 3, 2025 at 10:55 PM Rik van Riel <riel@...riel.com> wrote:
> > On Fri, 2025-01-03 at 19:39 +0100, Jann Horn wrote:
> > > 02fc2aa06e9e0ecdba3fe948cafe5892b72e86c0..3da645139748538daac70166618d
> > > 8ad95116eb74 100644
> > > --- a/arch/x86/include/asm/tlbflush.h
> > > +++ b/arch/x86/include/asm/tlbflush.h
> > > @@ -242,7 +242,7 @@ void flush_tlb_multi(const struct cpumask
> > > *cpumask,
> > >       flush_tlb_mm_range((vma)->vm_mm, start,
> > > end,                  \
> > >                          ((vma)->vm_flags &
> > > VM_HUGETLB)           \
> > >                               ?
> > > huge_page_shift(hstate_vma(vma))      \
> > > -                             : PAGE_SHIFT, false)
> > > +                             : PAGE_SHIFT, true)
> > >
> > >
> >
> > The code looks good, but should this macro get
> > a comment indicating that code that only frees
> > pages, but not page tables, should be calling
> > flush_tlb() instead?
> 
> Documentation/core-api/cachetlb.rst seems to be the common place
> that's supposed to document the rules - the macro I'm touching is just
> the x86 implementation. (The arm64 implementation also has some fairly
> extensive comments that say flush_tlb_range() "also invalidates any
> walk-cache entries associated with translations for the specified
> address range" while flush_tlb_page() "only invalidates a single,
> last-level page-table entry and therefore does not affect any
> walk-caches".) I wouldn't want to add yet more documentation for this
> API inside the X86 code. I guess it would make sense to add pointers
> from the x86 code to the documentation (and copy the details about
> last-level TLBs from the arm64 code into the docs).

Right, that documentation update might be nice.

Anyway, I'm picking up this patch for tip/x86/mm once -rc1 happens.

Thanks!