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SubjectRe: [PATCH 47/63] mm: numa: Do not batch handle PMD pages
On 10/07/2013 06:29 AM, Mel Gorman wrote:
> With the THP migration races closed it is still possible to occasionally
> see corruption. The problem is related to handling PMD pages in batch.
> When a page fault is handled it can be assumed that the page being
> faulted will also be flushed from the TLB. The same flushing does not
> happen when handling PMD pages in batch. Fixing is straight forward but
> there are a number of reasons not to
>
> 1. Multiple TLB flushes may have to be sent depending on what pages get
> migrated
> 2. The handling of PMDs in batch means that faults get accounted to
> the task that is handling the fault. While care is taken to only
> mark PMDs where the last CPU and PID match it can still have problems
> due to PID truncation when matching PIDs.
> 3. Batching on the PMD level may reduce faults but setting pmd_numa
> requires taking a heavy lock that can contend with THP migration
> and handling the fault requires the release/acquisition of the PTL
> for every page migrated. It's still pretty heavy.
>
> PMD batch handling is not something that people ever have been happy
> with. This patch removes it and later patches will deal with the
> additional fault overhead using more installigent migrate rate adaption.
>
> Signed-off-by: Mel Gorman <mgorman@suse.de>

Reviewed-by: Rik van Riel <riel@redhat.com>


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