On Mon, 3 Feb 2014, David Rientjes wrote: > On Tue, 4 Feb 2014, Joonsoo Kim wrote: > > > > > Okay. It can't fix your situation. Anyway, *normal* anon pages may be > > > > mapped > > > > and have positive page_count(), so your code such as > > > > '!page_mapping(page) && page_count(page)' makes compaction skip these > > > > *normal* > > > > anon pages and this is incorrect behaviour. > > > > > > > > > > So how does that work with migrate_page_move_mapping() which demands > > > page_count(page) == 1 and the get_page_unless_zero() in > > > __isolate_lru_page()? > > > > Before doing migrate_page_move_mapping(), try_to_unmap() is called so that > > all > > mapping is unmapped. Then, remained page_count() is 1 which is grabbed by > > __isolate_lru_page(). Am I missing something? > > > > Ah, good point. I wonder if we can get away with > page_count(page) - page_mapcount(page) > 1 to avoid the get_user_pages() > pin?
Something like that. But please go back to migrate_page_move_mapping() to factor in what it's additionally considering. Whether you can share code with it, I don't know - it has to do some things under a lock you cannot take at the preliminary stage - you haven't isolated or locked the page yet. There is a separate issue, that a mapping may supply its own non-default mapping->a_ops->migratepage(): can we assume that the page_counting is the same whatever migratepage() is in use? I'm not sure. If you stick to special-casing PageAnon pages, you won't face that issue; but your proposed change would be a lot more satisfying if we can convince ourselves that it's good for !PageAnon too. May need a trawl through the different migratepage() methods that exist in tree. Hugh -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
