Re: [PATCH v3 1/2] mm: migrate: prevent racy access to tlb_flush_pending

[Rik van Riel <riel@redhat.com>]

On Thu, 2017-07-27 at 04:40 -0700, Nadav Amit wrote:
> From: Nadav Amit <nadav.amit@gmail.com>
> 
> Setting and clearing mm->tlb_flush_pending can be performed by
> multiple
> threads, since mmap_sem may only be acquired for read in
> task_numa_work(). If this happens, tlb_flush_pending might be cleared
> while one of the threads still changes PTEs and batches TLB flushes.
> 
> This can lead to the same race between migration and
> change_protection_range() that led to the introduction of
> tlb_flush_pending. The result of this race was data corruption, which
> means that this patch also addresses a theoretically possible data
> corruption.
> 
> An actual data corruption was not observed, yet the race was
> was confirmed by adding assertion to check tlb_flush_pending is not
> set
> by two threads, adding artificial latency in
> change_protection_range()
> and using sysctl to reduce kernel.numa_balancing_scan_delay_ms.
> 
> Fixes: 20841405940e ("mm: fix TLB flush race between migration, and
> change_protection_range")
> 
> Cc: stable@vger.kernel.org
> 
> Signed-off-by: Nadav Amit <namit@vmware.com>
> Acked-by: Mel Gorman <mgorman@suse.de>
> 
Acked-by: Rik van Riel <riel@redhat.com>

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