Re: [PATCH 05/63] mm: Wait for THP migrations to complete duringNUMA hinting faults

[Rik van Riel]

On 10/07/2013 06:28 AM, Mel Gorman wrote:
> The locking for migrating THP is unusual. While normal page migration
> prevents parallel accesses using a migration PTE, THP migration relies on
> a combination of the page_table_lock, the page lock and the existance of
> the NUMA hinting PTE to guarantee safety but there is a bug in the scheme.
> 
> If a THP page is currently being migrated and another thread traps a
> fault on the same page it checks if the page is misplaced. If it is not,
> then pmd_numa is cleared. The problem is that it checks if the page is
> misplaced without holding the page lock meaning that the racing thread
> can be migrating the THP when the second thread clears the NUMA bit
> and faults a stale page.
> 
> This patch checks if the page is potentially being migrated and stalls
> using the lock_page if it is potentially being migrated before checking
> if the page is misplaced or not.
> 
> Cc: stable <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxx>

Acked-by: Rik van Riel <riel@xxxxxxxxxx>


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