Messages in this thread |  | | Date | Wed, 04 Dec 2013 09:33:53 -0500 | From | Rik van Riel <> | Subject | Re: [PATCH 14/15] mm: numa: Flush TLB if NUMA hinting faults race with PTE scan update |
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On 12/03/2013 06:46 PM, Mel Gorman wrote: > On Tue, Dec 03, 2013 at 06:07:06PM -0500, Rik van Riel wrote: >> On 12/03/2013 03:52 AM, Mel Gorman wrote: >>> NUMA PTE updates and NUMA PTE hinting faults can race against each other. The >>> setting of the NUMA bit defers the TLB flush to reduce overhead. NUMA >>> hinting faults do not flush the TLB as X86 at least does not cache TLB >>> entries for !present PTEs. However, in the event that the two race a NUMA >>> hinting fault may return with the TLB in an inconsistent state between >>> different processors. This patch detects potential for races between the >>> NUMA PTE scanner and fault handler and will flush the TLB for the affected >>> range if there is a race. >>> >>> Signed-off-by: Mel Gorman <mgorman@suse.de> >> >>> diff --git a/mm/migrate.c b/mm/migrate.c >>> index 5dfd552..ccc814b 100644 >>> --- a/mm/migrate.c >>> +++ b/mm/migrate.c >>> @@ -1662,6 +1662,39 @@ void wait_migrate_huge_page(struct anon_vma *anon_vma, pmd_t *pmd) >>> smp_rmb(); >>> } >>> >>> +unsigned long numa_fault_prepare(struct mm_struct *mm) >>> +{ >>> + /* Paired with task_numa_work */ >>> + smp_rmb(); >>> + return mm->numa_next_reset; >>> +} >> >> The patch that introduces mm->numa_next_reset, and the >> patch that increments it, seem to be missing from your >> series... >> > > Damn. s/numa_next_reset/numa_next_scan/ in that patch
How does that protect against the race?
Would it not be possible for task_numa_work to have a longer runtime than the numa fault?
In other words, task_numa_work can increment numa_next_scan before the numa fault starts, and still be doing its thing when numa_fault_commit is run...
At that point, numa_fault_commit will not be seeing an increment in numa_next_scan, and we are relying completely on the batched tlb flush by the change_prot_numa.
Is that scenario a problem, or is it ok?
And, why? :)
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