linux-kernel - Re: [PATCH v2 2/2] mm: fix missing cache flush for all tail pages of THP

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Date:   Tue, 25 Jan 2022 09:55:48 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Zi Yan <ziy@...dia.com>
Cc:     David Rientjes <rientjes@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Xiongchun duan <duanxiongchun@...edance.com>,
        Lars Persson <lars.persson@...s.com>
Subject: Re: [PATCH v2 2/2] mm: fix missing cache flush for all tail pages of THP

On Tue, Jan 25, 2022 at 3:22 AM Zi Yan <ziy@...dia.com> wrote:
>
> On 24 Jan 2022, at 13:11, David Rientjes wrote:
>
> > On Mon, 24 Jan 2022, Muchun Song wrote:
> >
> >> The D-cache maintenance inside move_to_new_page() only consider one page,
> >> there is still D-cache maintenance issue for tail pages of THP. Fix this
> >> by not using flush_dcache_folio() since it is not backportable.
> >>
> >
> > The mention of being backportable suggests that we should backport this,
> > likely to 4.14+.  So should it be marked as stable?
>
> Hmm, after more digging, I am not sure if the bug exists. For THP migration,
> flush_cache_range() is used in remove_migration_pmd(). The flush_dcache_page()
> was added by Lars Persson (cc’d) to solve the data corruption on MIPS[1],
> but THP migration is only enabled on x86_64, PPC_BOOK3S_64, and ARM64.

I only mention the THP case. After some more thinking, I think the HugeTLB
should also be considered, Right? The HugeTLB is enabled on arm, arm64,
mips, parisc, powerpc, riscv, s390 and sh.

>
> To make code more consistent, I guess flush_cache_range() in remove_migration_pmd()
> can be removed, since it is superseded by the flush_dcache_page() below.

>From my point of view, flush_cache_range() in remove_migration_pmd() is
a wrong usage, which cannot replace flush_dcache_page(). I think the commit
c2cc499c5bcf ("mm compaction: fix of improper cache flush in migration code")
, which is similar to the situation here, can offer more infos.

>
> The Fixes can be dropped. Let me know if I miss anything.
>
> >
> > That aside, should there be a follow-up patch that converts to using
> > flush_dcache_folio()?
>
> Are you suggesting to convert just this code or the entire move_to_new_page()
> to use folio? The latter might be more desirable, since the code will be
> more consistent.
>
>
> [1] https://lore.kernel.org/all/20190315083502.11849-1-larper@axis.com/T/#u
>