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    SubjectRe: [PATCH 11/12] vmscan: Write out dirty pages in batch
    > +	/*
    > + * XXX: This is the Holy Hand Grenade of PotentiallyInvalidMapping. As
    > + * the page lock has been dropped by ->writepage, that mapping could
    > + * be anything
    > + */

    Why is this an XXX comment?

    > + /*
    > + * Wait on writeback if requested to. This happens when
    > + * direct reclaiming a large contiguous area and the
    > + * first attempt to free a range of pages fails.
    > + */
    > + if (PageWriteback(page) && sync_writeback == PAGEOUT_IO_SYNC)
    > + wait_on_page_writeback(page);
    > +
    > + if (!PageWriteback(page)) {
    > + /* synchronous write or broken a_ops? */
    > + ClearPageReclaim(page);
    > + }

    how about:

    if (PageWriteback(page) {
    if (sync_writeback == PAGEOUT_IO_SYNC)
    wait_on_page_writeback(page);
    } else {
    /* synchronous write or broken a_ops? */
    ClearPageReclaim(page);
    }

    > if (!may_write_to_queue(mapping->backing_dev_info))
    > return PAGE_KEEP;
    >
    > /*
    > + * Clean a list of pages. It is expected that all the pages on page_list have been
    > + * locked as part of isolation from the LRU.

    A rather pointless line of 80 chars. I see the point for long string
    literals, but here's it's just a pain.

    > + *
    > + * XXX: Is there a problem with holding multiple page locks like this?

    I think there is. There's quite a few places that do hold multiple
    pages locked, but they always lock pages in increasing page->inxex order.
    Given that this locks basically in random order it could cause problems
    for those places.



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