linux-kernel - Re: [PATCH v11 07/14] mm: multi-gen LRU: exploit locality in rmap

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Date:   Thu, 16 Jun 2022 15:55:54 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     Barry Song <21cnbao@...il.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>, Andi Kleen <ak@...ux.intel.com>,
        Aneesh Kumar <aneesh.kumar@...ux.ibm.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Hillf Danton <hdanton@...a.com>, Jens Axboe <axboe@...nel.dk>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Michael Larabel <Michael@...haellarabel.com>,
        Michal Hocko <mhocko@...nel.org>,
        Mike Rapoport <rppt@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>, Vlastimil Babka <vbabka@...e.cz>,
        LAK <linux-arm-kernel@...ts.infradead.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>,
        Kernel Page Reclaim v2 <page-reclaim@...gle.com>,
        Brian Geffon <bgeffon@...gle.com>,
        Jan Alexander Steffens <heftig@...hlinux.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Steven Barrett <steven@...uorix.net>,
        Suleiman Souhlal <suleiman@...gle.com>,
        Daniel Byrne <djbyrne@....edu>,
        Donald Carr <d@...os-reins.com>,
        Holger Hoffstätte <holger@...lied-asynchrony.com>,
        Konstantin Kharlamov <Hi-Angel@...dex.ru>,
        Shuang Zhai <szhai2@...rochester.edu>,
        Sofia Trinh <sofia.trinh@....works>,
        Vaibhav Jain <vaibhav@...ux.ibm.com>, huzhanyuan@...o.com
Subject: Re: [PATCH v11 07/14] mm: multi-gen LRU: exploit locality in rmap

On Wed, Jun 8, 2022 at 4:46 PM Barry Song <21cnbao@...il.com> wrote:
>
> On Thu, Jun 9, 2022 at 3:52 AM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > On Tue, Jun 7, 2022 at 5:43 PM Barry Song <21cnbao@...il.com> wrote:
> > >
> > > Given we used to have a flush for clear pte young in LRU, right now we are
> > > moving to nop in almost all cases for the flush unless the address becomes
> > > young exactly after look_around and before ptep_clear_flush_young_notify.
> > > It means we are actually dropping flush. So the question is,  were we
> > > overcautious? we actually don't need the flush at all even without mglru?
> >
> > We stopped flushing the TLB on A bit clears on x86 back in 2014.
> >
> > See commit b13b1d2d8692 ("x86/mm: In the PTE swapout page reclaim case
> > clear the accessed bit instead of flushing the TLB").
>
> This is true for x86, RISC-V, powerpc and S390. but it is not true for
> most platforms.
>
> There was an attempt to do the same thing in arm64:
> https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1793830.html
> but arm64 still sent a nosync tlbi and depent on a deferred to dsb :
> https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1794484.html

Barry, you've already answered your own question.

Without commit 07509e10dcc7 arm64: pgtable: Fix pte_accessible():
   #define pte_accessible(mm, pte)        \
  -       (mm_tlb_flush_pending(mm) ? pte_present(pte) : pte_valid_young(pte))
  +       (mm_tlb_flush_pending(mm) ? pte_present(pte) : pte_valid(pte))

You missed all TLB flushes for PTEs that have gone through
ptep_test_and_clear_young() on the reclaim path. But most of the time,
you got away with it, only occasional app crashes:
https://lore.kernel.org/r/CAGsJ_4w6JjuG4rn2P=d974wBOUtXUUnaZKnx+-G6a8_mSROa+Q@mail.gmail.com/

Why?