linux-kernel - Re: [BUG?] X86 arch_tlbbatch_flush() seems to be lacking mm_tlb_flush

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Date:   Sat, 15 Oct 2022 16:47:16 -0700
From:   Linus Torvalds <torvalds@...uxfoundation.org>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Jann Horn <jannh@...gle.com>, Andy Lutomirski <luto@...nel.org>,
        Linux-MM <linux-mm@...ck.org>, Mel Gorman <mgorman@...e.de>,
        Rik van Riel <riel@...hat.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Sasha Levin <sasha.levin@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [BUG?] X86 arch_tlbbatch_flush() seems to be lacking
 mm_tlb_flush_nested() integration

On Fri, Oct 14, 2022 at 8:51 PM Nadav Amit <nadav.amit@...il.com> wrote:
>
> Unless I am missing something, flush_tlb_batched_pending() is would be
> called and do the flushing at this point, no?

Ahh, yes.

That seems to be doing the right thing, although looking a bit more at
it, I think it might be improved.

At least in the zap_pte_range() case, instead of doing a synchronous
TLB flush if there are pending batched flushes, it migth be better if
flush_tlb_batched_pending() would set the "need_flush_all" bit in the
mmu_gather structure.

That would possibly avoid that extra TLB flush entirely - since
*normally* fzap_page_range() will cause a TLB flush anyway.

Maybe it doesn't matter.

              Linus