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

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Date:   Sat, 29 Oct 2022 14:03:46 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Jann Horn <jannh@...gle.com>, X86 ML <x86@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        jroedel@...e.de, ubizjak@...il.com,
        Alistair Popple <apopple@...dia.com>
Subject: Re: [PATCH 01/13] mm: Update ptep_get_lockless()s comment

On Oct 29, 2022, at 1:56 PM, Nadav Amit <nadav.amit@...il.com> wrote:

> On Oct 29, 2022, at 1:15 PM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
>> (b) we could move the "page_remove_rmap()" into the "flush-and-free" path too
>> 
>> And (b) would be fairly easy - same model as that dirty bit patch,
>> just a 'do page_remove_rmap too' - except page_remove_rmap() wants the
>> vma as well (and we delay the TLB flush over multiple vma's, so it's
>> not just a "save vma in mmu_gather”).
> 
> (b) sounds reasonable and may potentially allow future performance
> improvements (batching, doing stuff without locks).
> 
> It does appear to break a potential hidden assumption that rmap is removed
> while the ptl is acquired (at least in the several instances I samples).
> Yet, anyhow page_vma_mapped_walk() checks the PTE before calling the
> function, so it should be fine.
> 
> I’ll give it a try.

I have just seen John’s and your emails. It seems (b) fell off. (a) is out
of my “zone”, and anyhow assuming it would not be solved soon, deferring
page_remove_rmap() might cause regressions.

(c) might be more intrusive and potentially induce overheads. If we need a
small backportable solution, I think the approach that I proposed (marking
the page dirty after the invalidation, before the PTL is released) is the
simplest one.

Please advise how to proceed.