On Sat, Oct 15, 2022 at 12:23 AM Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote: > On Fri, Oct 14, 2022 at 08:19:42PM +0200, Jann Horn wrote: > > Hi! > > > > 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 > > B: munmap() returns > > I think here we will serialize against anon_vma/i_mmap lock in > __do_munmap() -> unmap_region() -> free_pgtables() that A also holds. > > So I believe munmap() is safe, but MADV_DONTNEED (and its flavours) is not. shrink_folio_list() is not in a context that is operating on a specific MM; it is operating on a list of pages that might be mapped into different processes all over the system. So A has temporarily held those locks somewhere inside try_to_unmap_one(), but it will drop them before it reaches the point where it issues the batched TLB flush. And this batched TLB flush potentially covers multiple MMs at once; it is not targeted towards a specific MM, but towards all of the CPUs on which any of the touched MMs might be active.
