Re: [RFC PATCH 3/3] mm: unclutter THP migration

[Kirill A. Shutemov]

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