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Date: Thu, 02 Feb 2012 20:53:25 +0100 From: "Michal Nazarewicz" <mina86@...a86.com> To: "Marek Szyprowski" <m.szyprowski@...sung.com>, "Mel Gorman" <mel@....ul.ie> Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org, linux-mm@...ck.org, linaro-mm-sig@...ts.linaro.org, "'Kyungmin Park'" <kyungmin.park@...sung.com>, "'Russell King'" <linux@....linux.org.uk>, "'Andrew Morton'" <akpm@...ux-foundation.org>, "'KAMEZAWA Hiroyuki'" <kamezawa.hiroyu@...fujitsu.com>, "'Daniel Walker'" <dwalker@...eaurora.org>, "'Arnd Bergmann'" <arnd@...db.de>, "'Jesse Barker'" <jesse.barker@...aro.org>, "'Jonathan Corbet'" <corbet@....net>, "'Shariq Hasnain'" <shariq.hasnain@...aro.org>, "'Chunsang Jeong'" <chunsang.jeong@...aro.org>, "'Dave Hansen'" <dave@...ux.vnet.ibm.com>, "'Benjamin Gaignard'" <benjamin.gaignard@...aro.org> Subject: Re: [PATCH 02/15] mm: page_alloc: update migrate type of pages on pcp when isolating > 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@....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. 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(). 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. 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. -- Best regards, _ _ .o. | Liege of Serenely Enlightened Majesty of o' \,=./ `o ..o | Computer Science, Michał “mina86” Nazarewicz (o o) ooo +----<email/xmpp: mpn@...gle.com>--------------ooO--(_)--Ooo-- -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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