On Sat, Oct 15, 2022 at 12:29:57AM +0200, Jann Horn wrote: > 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. s/specific MM/specific page/ > So A has temporarily held those locks somewhere inside > try_to_unmap_one(), but it will drop them before it reaches the point inside try_to_unmap(), which handles all mappings of the page. > 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. But, yes, you are right. I thought that try_to_unmap_flush() called inside try_to_unmap() under the lock. -- Kiryl Shutsemau / Kirill A. Shutemov
