linux-kernel - Re: [PATCH 01/13] mm: Update ptep_get

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Message-ID: <Y1rdeFdYSBXOvTXz@hirez.programming.kicks-ass.net>
Date:   Thu, 27 Oct 2022 21:35:20 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Jann Horn <jannh@...gle.com>, John Hubbard <jhubbard@...dia.com>,
        x86@...nel.org, willy@...radead.org, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        aarcange@...hat.com, kirill.shutemov@...ux.intel.com,
        jroedel@...e.de, ubizjak@...il.com
Subject: Re: [PATCH 01/13] mm: Update ptep_get_lockless()s comment


Just two quick remarks; it's far to late to really think :-)

On Thu, Oct 27, 2022 at 11:13:55AM -0700, Linus Torvalds wrote:

> But "fullmm" is probably even stronger than "mmap write-lock" in that
> it should also mean "no other CPU can be actively using this" - either
> for hardware page table walking, or for GUP.

IIRC fullmm is really: this is the last user and we're taking the whole
mm down -- IOW exit().

> For example, MADV_DONTNEED does this all with just the mmap lock held
> for reading, so we *unless* we have that 'force_flush', we can
> 
>  (a) have another CPU continue to use the old stale TLB entry for quite a while
> 
>  (b) yet another CPU (that didn't have a TLB entry, or wanted to write
> to a read-only one ) could take a page fault, and install a *new* PTE
> entry in the same slot, all at the same time.
> 
> Now, that's clearly *very* confusing. But being confusing may not mean
> "wrong" - we're still delaying the free of the old entry, so there's
> no use-after-free.

Do we worry about CPU errata where things go side-ways if multiple CPUs
have inconsistent TLB state?