On Thu, Feb 02, 2012 at 08:53:25PM +0100, Michal Nazarewicz wrote:
On Tue, Jan 31, 2012 at 05:23:59PM +0100, Marek Szyprowski wrote:
Pages, which have incorrect migrate type on free finally causes pageblock migration type change from MIGRATE_CMA to MIGRATE_MOVABLE.
On Thu, 02 Feb 2012 13:47:29 +0100, Mel Gorman mel@csn.ul.ie wrote:
I'm not quite seeing this. In free_hot_cold_page(), the pageblock type is checked so the page private should be set to MIGRATE_CMA or MIGRATE_ISOLATE for the CMA area. It's not clear how this can change a pageblock to MIGRATE_MOVABLE in error.
Here's what I think may happen:
When drain_all_pages() is called, __free_one_page() is called for each page on pcp list with migrate type deducted from page_private() which is MIGRATE_CMA. This result in the page being put on MIGRATE_CMA freelist even though its pageblock's migrate type is MIGRATE_ISOLATE.
Ok, although it will only be allocated for MIGRATE_CMA-compatible requests so it is not a disaster.
When allocation happens and pcp list is empty, rmqueue_bulk() will get executed with migratetype argument set to MIGRATE_MOVABLE. It calls __rmqueue() to grab some pages and because the page described above is on MIGRATE_CMA freelist it may be returned back to rmqueue_bulk().
This will allocate the page from a pageblock we are trying to isolate pages from, but only for a movable page that can still be migrated. It does mean that CMA is doing more work than it should of course and the problem also impacts memory hot-remove. It's worse for memory hot-remove because potentially an UNMOVABLE page was allocated from a MIGRATE_ISOLATE pageblock.
But, pageblock's migrate type is not MIGRATE_CMA but MIGRATE_ISOLATE, so the following code:
#ifdef CONFIG_CMA if (is_pageblock_cma(page)) set_page_private(page, MIGRATE_CMA); else #endif set_page_private(page, migratetype);
will set it's private to MIGRATE_MOVABLE and in the end the page lands back on MIGRATE_MOVABLE pcp list but this time with page_private == MIGRATE_MOVABLE and not MIGRATE_CMA.
One more drain_all_pages() (which may happen since alloc_contig_range() calls set_migratetype_isolate() for each block) and next __rmqueue_fallback() may convert the whole pageblock to MIGRATE_MOVABLE.
I know, this sounds crazy and improbable, but I couldn't find an easier path to destruction. As you pointed, once the page is allocated, free_hot_cold_page() will do the right thing by reading pageblock's migrate type.
Ok, it's crazy but the problem is there.
Marek is currently experimenting with various patches including the following change:
#ifdef CONFIG_CMA int mt = get_pageblock_migratetype(page); if (is_migrate_cma(mt) || mt == MIGRATE_ISOLATE) set_page_private(page, mt); else #endif set_page_private(page, migratetype);
As a matter of fact, if __rmqueue() was changed to return migrate type of the freelist it took page from, we could avoid this get_pageblock_migratetype() all together. For now, however, I'd rather not go that way just yet -- I'll be happy to dig into it once CMA gets merged.
Ok, thanks for persisting with this.