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SubjectRe: [PATCH v9] mm: compaction: handle incorrect MIGRATE_UNMOVABLE type pageblocks
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On Mon, 04 Jun 2012 15:43:56 +0200, Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.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@google.com>--------------ooO--(_)--Ooo--
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