linux-kernel - Re: [RFC 01/20] mm/tlb: fix fullmm semantics

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Date:   Wed, 3 Feb 2021 09:44:59 +0000
From:   Will Deacon <will@...nel.org>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yu Zhao <yuzhao@...gle.com>, Nick Piggin <npiggin@...il.com>,
        X86 ML <x86@...nel.org>, Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 01/20] mm/tlb: fix fullmm semantics

On Tue, Feb 02, 2021 at 01:35:38PM -0800, Nadav Amit wrote:
> > On Feb 2, 2021, at 3:00 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > On Tue, Feb 02, 2021 at 01:32:36AM -0800, Nadav Amit wrote:
> >>> On Feb 1, 2021, at 3:36 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> >>> 
> >>> 
> >>> https://lkml.kernel.org/r/20210127235347.1402-1-will@kernel.org
> >> 
> >> I have seen this series, and applied my patches on it.
> >> 
> >> Despite Will’s patches, there were still inconsistencies between fullmm
> >> and need_flush_all.
> >> 
> >> Am I missing something?
> > 
> > I wasn't aware you were on top. I'll look again.
> 
> Looking on arm64’s tlb_flush() makes me think that there is currently a bug
> that this patch fixes. Arm64’s tlb_flush() does:
> 
>        /*
>         * If we're tearing down the address space then we only care about
>         * invalidating the walk-cache, since the ASID allocator won't
>         * reallocate our ASID without invalidating the entire TLB.
>         */
>        if (tlb->fullmm) {
>                if (!last_level)
>                        flush_tlb_mm(tlb->mm);
>                return;
>        } 
> 
> But currently tlb_mmu_finish() can mistakenly set fullmm incorrectly (if
> mm_tlb_flush_nested() is true), which might skip the TLB flush.

But in that case isn't 'freed_tables' set to 1, so 'last_level' will be
false and we'll do the flush in the code above?

Will