Re: [PATCH v5 1/3] mm: migrate: prevent racy access to tlb_flush_pending

[Minchan Kim <minchan@kernel.org>]

On Mon, Jul 31, 2017 at 09:43:23AM -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: Minchan Kim <minchan@kernel.org>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: stable@vger.kernel.org
> 
> Signed-off-by: Nadav Amit <namit@vmware.com>
> Acked-by: Mel Gorman <mgorman@suse.de>
Acked-by: Minchan Kim <minchan@kernel.org>

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