Re: [PATCH 02/15] mm: page_alloc: update migrate type of pages onpcp when isolating

From: Mel Gorman
Date: Fri Feb 03 2012 - 06:27:44 EST

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@xxxxxxxxx> 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.

Mel Gorman
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