linux-kernel - Re: [PATCH v9] mm: compaction: handle incorrect MIGRATE

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Date:	Mon, 04 Jun 2012 16:22:51 +0200
From:	"Michal Nazarewicz" <mina86@...a86.com>
To:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	"Bartlomiej Zolnierkiewicz" <b.zolnierkie@...sung.com>
Cc:	"Minchan Kim" <minchan@...nel.org>,
	"Hugh Dickins" <hughd@...gle.com>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Kyungmin Park" <kyungmin.park@...sung.com>,
	"Marek Szyprowski" <m.szyprowski@...sung.com>,
	"Mel Gorman" <mgorman@...e.de>, "Rik van Riel" <riel@...hat.com>,
	"Dave Jones" <davej@...hat.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Cong Wang" <amwang@...hat.com>,
	"Markus Trippelsdorf" <markus@...ppelsdorf.de>
Subject: Re: [PATCH v9] mm: compaction: handle incorrect MIGRATE_UNMOVABLE
 type pageblocks

On Mon, 04 Jun 2012 15:43:56 +0200, Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com> wrote:
> +/*
> + * Returns true if MIGRATE_UNMOVABLE pageblock can be successfully
> + * converted to MIGRATE_MOVABLE type, false otherwise.
> + */
> +static bool can_rescue_unmovable_pageblock(struct page *page, bool locked)
> +{
> +	unsigned long pfn, start_pfn, end_pfn;
> +	struct page *start_page, *end_page, *cursor_page;
> +
> +	pfn = page_to_pfn(page);
> +	start_pfn = pfn & ~(pageblock_nr_pages - 1);
> +	end_pfn = start_pfn + pageblock_nr_pages - 1;
> +
> +	start_page = pfn_to_page(start_pfn);
> +	end_page = pfn_to_page(end_pfn);
> +
> +	for (cursor_page = start_page, pfn = start_pfn; cursor_page <= end_page;
> +		pfn++, cursor_page++) {
> +		struct zone *zone = page_zone(start_page);
> +		unsigned long flags;
> +
> +		if (!pfn_valid_within(pfn))
> +			continue;
> +
> +		/* Do not deal with pageblocks that overlap zones */
> +		if (page_zone(cursor_page) != zone)
> +			return false;
> +
> +		if (!locked)
> +			spin_lock_irqsave(&zone->lock, flags);
> +
> +		if (PageBuddy(cursor_page)) {
> +			int order = page_order(cursor_page);
>-/* Returns true if the page is within a block suitable for migration to */
> -static bool suitable_migration_target(struct page *page)
> +			pfn += (1 << order) - 1;
> +			cursor_page += (1 << order) - 1;
> +
> +			if (!locked)
> +				spin_unlock_irqrestore(&zone->lock, flags);
> +			continue;
> +		} else if (page_count(cursor_page) == 0 ||
> +			   PageLRU(cursor_page)) {
> +			if (!locked)
> +				spin_unlock_irqrestore(&zone->lock, flags);
> +			continue;
> +		}
> +
> +		if (!locked)
> +			spin_unlock_irqrestore(&zone->lock, flags);

spin_unlock in three spaces is ugly.  How about adding a flag that holds the
result of the function which you use as for loop condition and you set it to
false inside an additional else clause?  Eg.:

	bool result = true;
	for (...; result && cursor_page <= end_page; ...) {
		...
		if (!pfn_valid_within(pfn)) continue;
		if (page_zone(cursor_page) != zone) return false;
		if (!locked) spin_lock_irqsave(...);
		
		if (PageBuddy(...)) {
			...
		} else if (page_count(cursor_page) == 0 ||
			   PageLRU(cursor_page)) {
			...
		} else {
			result = false;
		}
		if (!locked) spin_unlock_irqsave(...);
	}
	return result;

> +		return false;
> +	}
> +
> +	return true;
> +}

How do you make sure that a page is not allocated while this runs?  Or you just
don't care?  Not that even with zone lock, page may be allocated from pcp list
on (another) CPU.

-- 
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--
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