Re: [PATCH 27/50] mm: numa: Scan pages with elevated page_mapcount

[Mel Gorman]

On Thu, Sep 12, 2013 at 10:10:13AM +0800, Hillf Danton wrote:
> Hillo Mel
> 
> On Tue, Sep 10, 2013 at 5:32 PM, Mel Gorman <mgorman@xxxxxxx> wrote:
> > Currently automatic NUMA balancing is unable to distinguish between false
> > shared versus private pages except by ignoring pages with an elevated
> > page_mapcount entirely. This avoids shared pages bouncing between the
> > nodes whose task is using them but that is ignored quite a lot of data.
> >
> > This patch kicks away the training wheels in preparation for adding support
> > for identifying shared/private pages is now in place. The ordering is so
> > that the impact of the shared/private detection can be easily measured. Note
> > that the patch does not migrate shared, file-backed within vmas marked
> > VM_EXEC as these are generally shared library pages. Migrating such pages
> > is not beneficial as there is an expectation they are read-shared between
> > caches and iTLB and iCache pressure is generally low.
> >
> > Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
> > ---
> [...]
> > @@ -1658,13 +1660,6 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
> >         int page_lru = page_is_file_cache(page);
> >
> >         /*
> > -        * Don't migrate pages that are mapped in multiple processes.
> > -        * TODO: Handle false sharing detection instead of this hammer
> > -        */
> > -       if (page_mapcount(page) != 1)
> > -               goto out_dropref;
> > -
> Is there rmap walk when migrating THP?
> 

Should not be necessary for THP.

-- 
Mel Gorman
SUSE Labs
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