linux-kernel - Re: [PATCH 6/6] mm: proc: Avoid fullmm flush for young/dirty bit toggling

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Date:   Fri, 20 Nov 2020 09:41:54 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Will Deacon <will@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Android Kernel Team <kernel-team@...roid.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Yu Zhao <yuzhao@...gle.com>, Minchan Kim <minchan@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 6/6] mm: proc: Avoid fullmm flush for young/dirty bit toggling

On Fri, Nov 20, 2020 at 6:36 AM Will Deacon <will@...nel.org> wrote:
>
> Ensure that TLB invalidation is performed after updating soft-dirty
> entries via clear_refs_write() by using the non-fullmm API to MMU gather.

This code sequence looks bogus to begin with.

It does that

                tlb_gather_mmu(&tlb, mm, 0, -1);
     ..
                tlb_finish_mmu(&tlb, 0, -1);

around the loop (all, your patch series changes those arguments), but
it doesn't actually use "tlb" anywhere inside the loop itself that I
can see.

Yeah., yeah, it sets the flush_pending thing etc, but that still
sounds fundamentally wrong. It should do the proper range adjustments
if/when it actually wals the range. No?

If I read this all right, it will do a full TLB flush even when it
doesn't do anything (eg CLEAR_REFS_SOFT_DIRTY with no softdirty
pages).

So this looks all kinds of bogus. Not your patch, but the code it patches.

               Linus