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From:   Jann Horn <jannh@google.com>
Date:   Mon, 17 Oct 2022 12:56:37 +0200
Message-ID: <CAG48ez3bPLVA2tqoYUT_mKp3u_yUg5QXx7AmxASrTE=jxnNWJg@mail.gmail.com>
Subject: Re: [BUG?] X86 arch_tlbbatch_flush() seems to be lacking
 mm_tlb_flush_nested() integration
To:     Nadav Amit <nadav.amit@gmail.com>
Cc:     Andy Lutomirski <luto@kernel.org>, Linux-MM <linux-mm@kvack.org>,
        Mel Gorman <mgorman@suse.de>, Rik van Riel <riel@redhat.com>,
        kernel list <linux-kernel@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Ingo Molnar <mingo@kernel.org>,
        Sasha Levin <sasha.levin@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Linus Torvalds <torvalds@linuxfoundation.org>
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On Sat, Oct 15, 2022 at 5:51 AM Nadav Amit <nadav.amit@gmail.com> wrote:
> On Oct 14, 2022, at 9:19 PM, Jann Horn <jannh@google.com> wrote:
> > I haven't actually managed to reproduce this behavior, so maybe I'm
> > just misunderstanding how this works; but I think the
> > arch_tlbbatch_flush() path for batched TLB flushing in vmscan ought to
> > have some kind of integration with mm_tlb_flush_nested().
> >
> > I think that currently, the following race could happen:
> >
> > [initial situation: page P is mapped into a page table of task B, but
> > the page is not referenced, the PTE's A/D bits are clear]
> > A: vmscan begins
> > A: vmscan looks at P and P's PTEs, and concludes that P is not currently in use
> > B: reads from P through the PTE, setting the Accessed bit and creating
> > a TLB entry
> > A: vmscan enters try_to_unmap_one()
> > A: try_to_unmap_one() calls should_defer_flush(), which returns true
> > A: try_to_unmap_one() removes the PTE and queues a TLB flush
> > (arch_tlbbatch_add_mm())
> > A: try_to_unmap_one() returns, try_to_unmap() returns to shrink_folio_list()
> > B: calls munmap() on the VMA that mapped P
> > B: no PTEs are removed, so no TLB flush happens
>
> Unless I am missing something, flush_tlb_batched_pending() is would be
> called and do the flushing at this point, no?

Ooooh! Thanks, I missed that.