On Sat, Feb 01, 2014 at 09:46:26PM -0800, David Rientjes wrote:
> Page migration will fail for memory that is pinned in memory with, for
> example, get_user_pages(). In this case, it is unnecessary to take
> zone->lru_lock or isolating the page and passing it to page migration
> which will ultimately fail.
>
> This is a racy check, the page can still change from under us, but in
> that case we'll just fail later when attempting to move the page.
>
> This avoids very expensive memory compaction when faulting transparent
> hugepages after pinning a lot of memory with a Mellanox driver.
>
> On a 128GB machine and pinning ~120GB of memory, before this patch we
> see the enormous disparity in the number of page migration failures
> because of the pinning (from /proc/vmstat):
>
> compact_blocks_moved 7609
> compact_pages_moved 3431
> compact_pagemigrate_failed 133219
> compact_stall 13
>
> After the patch, it is much more efficient:
>
> compact_blocks_moved 7998
> compact_pages_moved 6403
> compact_pagemigrate_failed 3
> compact_stall 15
>
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
> ---
> mm/compaction.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -578,6 +578,14 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
> continue;
> }
>
> + /*
> + * Migration will fail if an anonymous page is pinned in memory,
> + * so avoid taking zone->lru_lock and isolating it unnecessarily
> + * in an admittedly racy check.
> + */
> + if (!page_mapping(page) && page_count(page))
> + continue;
> +
Are you sure about this? The page_count check migration does is this
int expected_count = 1 + extra_count;
if (!mapping) {
if (page_count(page) != expected_count)
return -EAGAIN;
return MIGRATEPAGE_SUCCESS;
}
spin_lock_irq(&mapping->tree_lock);
pslot = radix_tree_lookup_slot(&mapping->page_tree,
page_index(page));
expected_count += 1 + page_has_private(page);
Migration expects and can migrate pages with no mapping and a page count
but you are now skipping them. I think you may have intended to split
migrations page count into a helper or copy the logic.
--
Mel Gorman
SUSE Labs
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