Re: Potential race in TLB flush batching?

[Mel Gorman <mgorman@suse.de>]

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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