Re: [PATCH 2/2] mm: support MIGRATE_DISCARD

[Mel Gorman <mgorman@suse.de>]

On Thu, Sep 06, 2012 at 09:29:35AM +0100, Mel Gorman wrote:
> On Thu, Sep 06, 2012 at 02:31:12PM +0900, Minchan Kim wrote:
> > Hi Mel,
> > 
> > On Wed, Sep 05, 2012 at 11:56:11AM +0100, Mel Gorman wrote:
> > > On Wed, Sep 05, 2012 at 05:11:13PM +0900, Minchan Kim wrote:
> > > > This patch introudes MIGRATE_DISCARD mode in migration.
> > > > It drops *clean cache pages* instead of migration so that
> > > > migration latency could be reduced by avoiding (memcpy + page remapping).
> > > > It's useful for CMA because latency of migration is very important rather
> > > > than eviction of background processes's workingset. In addition, it needs
> > > > less free pages for migration targets so it could avoid memory reclaiming
> > > > to get free pages, which is another factor increase latency.
> > > > 
> > > 
> > > Bah, this was released while I was reviewing the older version. I did
> > > not read this one as closely but I see the enum problems have gone away
> > > at least. I'd still prefer if CMA had an additional helper to discard
> > > some pages with shrink_page_list() and migrate the remaining pages with
> > > migrate_pages(). That would remove the need to add a MIGRATE_DISCARD
> > > migrate mode at all.
> > 
> > I am not convinced with your point. What's the benefit on separating
> > reclaim and migration? For just removing MIGRATE_DISCARD mode?
> 
> Maintainability. There are reclaim functions and there are migration
> functions. Your patch takes migrate_pages() and makes it partially a
> reclaim function mixing up the responsibilities of migrate.c and vmscan.c.
> 
> > I don't think it's not bad because my implementation is very simple(maybe
> > it's much simpler than separating reclaim and migration) and
> > could be used by others like memory-hotplug in future.
> 
> They could also have used the helper function from CMA that takes a list
> of pages, reclaims some and migrates other.
> 

I also do not accept that your approach is inherently simpler than what I
proposed to you. This is not tested at all but it should be functionally
similar to both your patches except that it keeps the responsibility for
reclaim in vmscan.c

Your diffstats are

8 files changed, 39 insertions(+), 36 deletions(-)
3 files changed, 46 insertions(+), 4 deletions(-)

Mine is

 3 files changed, 32 insertions(+), 5 deletions(-)

Fewer files changed and fewer lines inserted.

---8<---
mm: cma: Discard clean pages during contiguous allocation instead of migration

This patch drops clean cache pages instead of migration during
alloc_contig_range() to minimise allocation latency by reducing the amount
of migration is necessary. It's useful for CMA because latency of migration
is more important than evicting the background processes working set.

Prototype-not-signed-off-but-feel-free-to-pick-up-and-test
---
 mm/internal.h   |    1 +
 mm/page_alloc.c |    2 ++
 mm/vmscan.c     |   34 +++++++++++++++++++++++++++++-----
 3 files changed, 32 insertions(+), 5 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index b8c91b3..6d4bdf9 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -356,3 +356,4 @@ extern unsigned long vm_mmap_pgoff(struct file *, unsigned long,
         unsigned long, unsigned long);
 
 extern void set_pageblock_order(void);
+unsigned long reclaim_clean_pages_from_list(struct list_head *page_list);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c66fb87..977bdb2 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5670,6 +5670,8 @@ static int __alloc_contig_migrate_range(unsigned long start, unsigned long end)
 			break;
 		}
 
+		reclaim_clean_pages_from_list(&cc.migratepages);
+
 		ret = migrate_pages(&cc.migratepages,
 				    __alloc_contig_migrate_alloc,
 				    0, false, MIGRATE_SYNC);
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 8d01243..ccf7bc2 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -703,7 +703,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 			goto keep;
 
 		VM_BUG_ON(PageActive(page));
-		VM_BUG_ON(page_zone(page) != zone);
+		VM_BUG_ON(zone && page_zone(page) != zone);
 
 		sc->nr_scanned++;
 
@@ -817,7 +817,9 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 				 * except we already have the page isolated
 				 * and know it's dirty
 				 */
-				inc_zone_page_state(page, NR_VMSCAN_IMMEDIATE);
+				if (zone)
+					inc_zone_page_state(page,
+							NR_VMSCAN_IMMEDIATE);
 				SetPageReclaim(page);
 
 				goto keep_locked;
@@ -947,7 +949,7 @@ keep:
 	 * back off and wait for congestion to clear because further reclaim
 	 * will encounter the same problem
 	 */
-	if (nr_dirty && nr_dirty == nr_congested && global_reclaim(sc))
+	if (zone && nr_dirty && nr_dirty == nr_congested && global_reclaim(sc))
 		zone_set_flag(zone, ZONE_CONGESTED);
 
 	free_hot_cold_page_list(&free_pages, 1);
@@ -955,11 +957,33 @@ keep:
 	list_splice(&ret_pages, page_list);
 	count_vm_events(PGACTIVATE, pgactivate);
 	mem_cgroup_uncharge_end();
-	*ret_nr_dirty += nr_dirty;
-	*ret_nr_writeback += nr_writeback;
+	if (ret_nr_dirty)
+		*ret_nr_dirty += nr_dirty;
+	if (ret_nr_writeback)
+		*ret_nr_writeback += nr_writeback;
 	return nr_reclaimed;
 }
 
+unsigned long reclaim_clean_pages_from_list(struct list_head *page_list)
+{
+	struct scan_control sc = {
+		.gfp_mask = GFP_KERNEL,
+		.priority = DEF_PRIORITY,
+	};
+	unsigned long ret;
+	struct page *page, *next;
+	LIST_HEAD(clean_pages);
+
+	list_for_each_entry_safe(page, next, page_list, lru) {
+		if (page_is_file_cache(page) && !PageDirty(page))
+			list_move(&page->lru, &clean_pages);
+	}
+
+	ret = shrink_page_list(&clean_pages, NULL, &sc, NULL, NULL);
+	list_splice(&clean_pages, page_list);
+	return ret;
+}
+
 /*
  * Attempt to remove the specified page from its LRU.  Only take this page
  * if it is of the appropriate PageActive status.  Pages which are being

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