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dkim=pass header.d=google.com header.s=20210112 header.b="KG/CPftv"; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf09.hostedemail.com: domain of jannh@google.com designates 209.85.166.51 as permitted sender) smtp.mailfrom=jannh@google.com X-Stat-Signature: nz5tz3639gwewa75zb3wmwokc54iza83 X-HE-Tag: 1666004234-862066 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Sat, Oct 15, 2022 at 5:51 AM Nadav Amit wrote: > On Oct 14, 2022, at 9:19 PM, Jann Horn 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.