On Thu, 6 Feb 2014, Joonsoo Kim wrote: > On Wed, Feb 05, 2014 at 12:56:40PM -0800, Hugh Dickins wrote: > > On Tue, 4 Feb 2014, 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_pages_moved 8450 > > > compact_pagemigrate_failed 15614415 > > > > > > 0.05% of pages isolated are successfully migrated and explicitly > > > triggering memory compaction takes 102 seconds. After the patch: > > > > > > compact_pages_moved 9197 > > > compact_pagemigrate_failed 7 > > > > > > 99.9% of pages isolated are now successfully migrated in this > > > configuration and memory compaction takes less than one second. > > > > > > Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx> > > > --- > > > v2: address page count issue per Joonsoo > > > > > > mm/compaction.c | 9 +++++++++ > > > 1 file changed, 9 insertions(+) > > > > > > diff --git a/mm/compaction.c b/mm/compaction.c > > > --- a/mm/compaction.c > > > +++ b/mm/compaction.c > > > @@ -578,6 +578,15 @@ 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 lru_lock and isolating it unnecessarily in an > > > + * admittedly racy check. > > > + */ > > > + if (!page_mapping(page) && > > > + page_count(page) > page_mapcount(page)) > > > + continue; > > > + > > > /* Check if it is ok to still hold the lock */ > > > locked = compact_checklock_irqsave(&zone->lru_lock, &flags, > > > > Much better, maybe good enough as an internal patch to fix a particular > > problem you're seeing; but not yet good enough to go upstream. > > > > Anonymous pages are not the only pages which might be pinned, > > and your test doesn't mention PageAnon, so does not match your comment. > > > > I've remembered is_page_cache_freeable() in mm/vmscan.c, which gives > > more assurance that a page_count - page_has_private test is appropriate, > > whatever the filesystem and migrate method to be used. > > > > So I think the test you're looking for is > > > > pincount = page_count(page) - page_mapcount(page); > > if (page_mapping(page)) > > pincount -= 1 + page_has_private(page); > > if (pincount > 0) > > continue; > > > > but please cross-check and test that out, it's easy to be off-by-one etc. > > Hello, Hugh. > > I don't think that this is right. > One of migratepage function, aio_migratepage(), pass extra count 1 to > migrate_page_move_mapping(). So it can be migrated when pincount == 1 in > above test. > > I think that we should not be aggressive here. This is just for prediction > so that it is better not to skip apropriate pages at most. Just for anon case > that we are sure easily is the right solution for me. Interesting, thank you for the pointer. That's a pity! I hope that later on we can modify fs/aio.c to set PagePrivate on ring pages, revert the extra argument to migrate_page_move_mapping(), and then let it appear the same as the other filesystems (but lacking a writepage, reclaim won't try to free the pages). But that's "later on" and may prove impossible in the implementation. I agree it's beyond the scope of David's patch, and so only anonymous should be dealt with in this way at present. And since page_mapping() is non-NULL on PageAnon PageSwapCache pages, those will fall through David's test and go on to try migration: which is the correct default. Although we could add code to handle pinned swapcache, it would be rather an ugly excrescence, until the case gets handled naturally when proper page_mapping() support is added later. Okay, to David's current patch Acked-by: Hugh Dickins <hughd@xxxxxxxxxx> though I'd like to hear whether Mel is happy with it. Hugh -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>