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    SubjectRe: [PATCH 05/11] mm: compaction: Determine if dirty pages can be migrated without blocking within ->migratepage
    On Wed, 14 Dec 2011 15:41:27 +0000
    Mel Gorman <mgorman@suse.de> wrote:

    > Asynchronous compaction is used when allocating transparent hugepages
    > to avoid blocking for long periods of time. Due to reports of
    > stalling, there was a debate on disabling synchronous compaction
    > but this severely impacted allocation success rates. Part of the
    > reason was that many dirty pages are skipped in asynchronous compaction
    > by the following check;
    >
    > if (PageDirty(page) && !sync &&
    > mapping->a_ops->migratepage != migrate_page)
    > rc = -EBUSY;
    >
    > This skips over all mapping aops using buffer_migrate_page()
    > even though it is possible to migrate some of these pages without
    > blocking. This patch updates the ->migratepage callback with a "sync"
    > parameter. It is the responsibility of the callback to fail gracefully
    > if migration would block.
    >
    > ...
    >
    > @@ -259,6 +309,19 @@ static int migrate_page_move_mapping(struct address_space *mapping,
    > }
    >
    > /*
    > + * In the async migration case of moving a page with buffers, lock the
    > + * buffers using trylock before the mapping is moved. If the mapping
    > + * was moved, we later failed to lock the buffers and could not move
    > + * the mapping back due to an elevated page count, we would have to
    > + * block waiting on other references to be dropped.
    > + */
    > + if (!sync && head && !buffer_migrate_lock_buffers(head, sync)) {

    Once it has been established that "sync" is true, I find it clearer to
    pass in plain old "true" to buffer_migrate_lock_buffers(). Minor point.



    I hadn't paid a lot of attention to buffer_migrate_page() before.
    Scary function. I'm rather worried about its interactions with ext3
    journal commit which locks buffers then plays with them while leaving
    the page unlocked. How vigorously has this been whitebox-tested?


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