Re: [PATCHv2] CMA/HOTPLUG: clear buffer-head lru before page migration

[Joonsoo Kim]

On Fri, Aug 01, 2014 at 09:12:06AM +0900, Gioh Kim wrote:
> 
> 
> 2014-08-01 ìì 7:57, Andrew Morton ì ê:
> >On Thu, 31 Jul 2014 11:22:35 +0900 Gioh Kim <gioh.kim@xxxxxxx> wrote:
> >
> >>The previous PATCH inserts invalidate_bh_lrus() only into CMA code.
> >>HOTPLUG needs also dropping bh of lru.
> >>So v2 inserts invalidate_bh_lrus() into both of CMA and HOTPLUG.
> >>
> >>
> >>---------------------------- 8< ----------------------------
> >>The bh must be free to migrate a page at which bh is mapped.
> >>The reference count of bh is increased when it is installed
> >>into lru so that the bh of lru must be freed before migrating the page.
> >>
> >>This frees every bh of lru. We could free only bh of migrating page.
> >>But searching lru sometimes costs more than invalidating entire lru.
> >>
> >>Signed-off-by: Gioh Kim <gioh.kim@xxxxxxx>
> >>Acked-by: Michal Nazarewicz <mina86@xxxxxxxxxx>
> >>---
> >>  mm/memory_hotplug.c |    1 +
> >>  mm/page_alloc.c     |    2 ++
> >>  2 files changed, 3 insertions(+)
> >>
> >>diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> >>index a3797d3..1c5454f 100644
> >>--- a/mm/memory_hotplug.c
> >>+++ b/mm/memory_hotplug.c
> >>@@ -1672,6 +1672,7 @@ repeat:
> >>                 lru_add_drain_all();
> >>                 cond_resched();
> >>                 drain_all_pages();
> >>+               invalidate_bh_lrus();
> >
> >Both of these calls should have a comment explaining why
> >invalidate_bh_lrus() is being called.
> >
> >>         }
> >>
> >>         pfn = scan_movable_pages(start_pfn, end_pfn);
> >>diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> >>index b99643d4..c00dedf 100644
> >>--- a/mm/page_alloc.c
> >>+++ b/mm/page_alloc.c
> >>@@ -6369,6 +6369,8 @@ int alloc_contig_range(unsigned long start, unsigned long end,
> >>         if (ret)
> >>                 return ret;
> >>
> >>+       invalidate_bh_lrus();
> >>+
> >>         ret = __alloc_contig_migrate_range(&cc, start, end);
> >>         if (ret)
> >>                 goto done;
> >
> >I do feel that this change is likely to be beneficial, but I don't want
> >to apply such a patch until I know what its effects are upon all
> >alloc_contig_range() callers.  Especially hugetlb.
> 
> I'm very sorry to hear that.
> How can I check the effects?
> 

Hello, Gioh.

As you know, I generally agree this patch, but, I want to know that
this patch really fixes your problem. There is some time difference
between invalidate_bh_lrus() and migrate_page() so that the bh of the
migrating page could be re-installed on bh lru again. Any
remarkable success rate changes? If this time gap is critical, we
should put this invalidation logic on other place.

Thanks.
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