Re: [BUG?] X86 arch_tlbbatch_flush() seems to be lacking mm_tlb_flush_nested() integration

[Jann Horn]

On Sat, Oct 15, 2022 at 5:51 AM Nadav Amit <nadav.amit@xxxxxxxxx> wrote:
> On Oct 14, 2022, at 9:19 PM, Jann Horn <jannh@xxxxxxxxxx> 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.