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

Joonsoo Kim
2014-08-06T07:30:00Z

On Fri, Aug 01, 2014 at 09:12:06AM +0900, Gioh Kim wrote:
>
>
> 2014-08-01 오전 7:57, Andrew Morton 쓴 글:
> >On Thu, 31 Jul 2014 11:22:35 +0900 Gioh Kim <gioh.kim@lge.com> 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@lge.com>
> >>Acked-by: Michal Nazarewicz <mina86@mina86.com>
> >>---
> >> 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.