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    SubjectRe: [PATCH 11/12] vmscan: Write out dirty pages in batch
    On Tue, Jun 15, 2010 at 06:53:41AM -0400, Christoph Hellwig wrote:
    > > + /*
    > > + * 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?
    >

    With the page lock released, the mapping may be no longer valid. Nick
    posted a patch in relation to it that I need to look at. The comment was
    because Andrew highlight that this was buggy and I wanted to make sure I
    didn't forget about it.

    > > + /*
    > > + * 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);
    > }
    >

    Sure, that's tidier.

    > > 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.
    >

    I'll trim it.

    > > + *
    > > + * 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.
    >

    Hmm, ok. In that case, I'll have to release the locks on the list and
    reacquire them. It was something I would have preferred to avoid. Thanks

    --
    Mel Gorman
    Part-time Phd Student Linux Technology Center
    University of Limerick IBM Dublin Software Lab


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