Re: [PATCH 4/5] mm: compaction: Determine if dirty pages can bemigreated without blocking within ->migratepage

[Mel Gorman]

On Sat, Nov 19, 2011 at 04:59:10PM +0800, Nai Xia wrote:
> > <SNIP>
> > @@ -453,13 +494,18 @@ int buffer_migrate_page(struct address_space *mapping,
> >        if (rc)
> >                return rc;
> >
> > -       bh = head;
> > -       do {
> > -               get_bh(bh);
> > -               lock_buffer(bh);
> > -               bh = bh->b_this_page;
> > -
> > -       } while (bh != head);
> > +       if (!buffer_migrate_lock_buffers(head, sync)) {
> > +               /*
> > +                * We have to revert the radix tree update. If this returns
> > +                * non-zero, it either means that the page count changed
> > +                * which "can't happen" or the slot changed from underneath
> > +                * us in which case someone operated on a page that did not
> > +                * have buffers fully migrated which is alarming so warn
> > +                * that it happened.
> > +                */
> > +               WARN_ON(migrate_page_move_mapping(mapping, page, newpage));
> > +               return -EBUSY;
> 
> If this migrate_page_move_mapping() really fails, seems disk IO will be needed
> to bring the previously already cached page back,

Aside from that, I couldn't see a way of handling the case where the
page had an elevated count due to a speculative lookup.

> I wonder if we should make the
> double check for the two conditions of "page refs is ok " and "all bh
> trylocked"
> before doing radix_tree_replace_slot() ? which I think does not
> involve IO on the
> error path.
> 

I reached the same conclusion when figuring out how to backout of the
the elevated page count case. In an updated patch,
migrate_page_move_mapping() returns with buffers locked in the async
case.

-- 
Mel Gorman
SUSE Labs
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