On Fri, Dec 08, 2017 at 05:15:59PM +0100, Michal Hocko wrote: > From: Michal Hocko <mhocko@xxxxxxxx> > > THP migration is hacked into the generic migration with rather > surprising semantic. The migration allocation callback is supposed to > check whether the THP can be migrated at once and if that is not the > case then it allocates a simple page to migrate. unmap_and_move then > fixes that up by spliting the THP into small pages while moving the > head page to the newly allocated order-0 page. Remaning pages are moved > to the LRU list by split_huge_page. The same happens if the THP > allocation fails. This is really ugly and error prone [1]. > > I also believe that split_huge_page to the LRU lists is inherently > wrong because all tail pages are not migrated. Some callers will just > work around that by retrying (e.g. memory hotplug). There are other > pfn walkers which are simply broken though. e.g. madvise_inject_error > will migrate head and then advances next pfn by the huge page size. > do_move_page_to_node_array, queue_pages_range (migrate_pages, mbind), > will simply split the THP before migration if the THP migration is not > supported then falls back to single page migration but it doesn't handle > tail pages if the THP migration path is not able to allocate a fresh > THP so we end up with ENOMEM and fail the whole migration which is > a questionable behavior. Page compaction doesn't try to migrate large > pages so it should be immune. > > This patch tries to unclutter the situation by moving the special THP > handling up to the migrate_pages layer where it actually belongs. We > simply split the THP page into the existing list if unmap_and_move fails > with ENOMEM and retry. So we will _always_ migrate all THP subpages and > specific migrate_pages users do not have to deal with this case in a > special way. > > [1] http://lkml.kernel.org/r/20171121021855.50525-1-zi.yan@xxxxxxxx > > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx> Looks good to me. Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
