On Tue, Jul 11, 2017 at 01:06:48PM -0700, Nadav Amit wrote:
> > +/*
> > + * Reclaim batches unmaps pages under the PTL but does not flush the TLB
> > + * TLB prior to releasing the PTL. It's possible a parallel mprotect or
> > + * munmap can race between reclaim unmapping the page and flushing the
> > + * page. If this race occurs, it potentially allows access to data via
> > + * a stale TLB entry. Tracking all mm's that have TLB batching pending
> > + * would be expensive during reclaim so instead track whether TLB batching
> > + * occured in the past and if so then do a full mm flush here. This will
> > + * cost one additional flush per reclaim cycle paid by the first munmap or
> > + * mprotect. This assumes it's called under the PTL to synchronise access
> > + * to mm->tlb_flush_batched.
> > + */
> > +void flush_tlb_batched_pending(struct mm_struct *mm)
> > +{
> > + if (mm->tlb_flush_batched) {
> > + flush_tlb_mm(mm);
> > + mm->tlb_flush_batched = false;
> > + }
> > +}
> > #else
> > static void set_tlb_ubc_flush_pending(struct mm_struct *mm, bool writable)
> > {
>
> I don???t know what is exactly the invariant that is kept, so it is hard for
> me to figure out all sort of questions:
>
> Should pte_accessible return true if mm->tlb_flush_batch==true ?
>
It shouldn't be necessary. The contexts where we hit the path are
uprobes: elevated page count so no parallel reclaim
dax: PTEs are not mapping that would be reclaimed
hugetlbfs: Not reclaimed
ksm: holds page lock and elevates count so cannot race with reclaim
cow: at the time of the flush, the page count is elevated so cannot race with reclaim
page_mkclean: only concerned with marking existing ptes clean but in any
case, the batching flushes the TLB before issueing any IO so there
isn't space for a stable TLB entry to be used for something bad.
> Does madvise_free_pte_range need to be modified as well?
>
Yes, I noticed that out shortly after sending the first version and
commented upon it.
> How will future code not break anything?
>
I can't really answer that without a crystal ball. Code dealing with page
table updates would need to take some care if it can race with parallel
reclaim.
--
Mel Gorman
SUSE Labs
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