Re: [PATCH v3 1/3] mm/free_pcppages_bulk: update pcp->count inside

[Aaron Lu <aaron.lu@intel.com>]

On Tue, Feb 27, 2018 at 09:56:13AM +0800, Aaron Lu wrote:
> On Mon, Feb 26, 2018 at 01:48:14PM -0800, David Rientjes wrote:
> > On Mon, 26 Feb 2018, Aaron Lu wrote:
> > 
> > > Matthew Wilcox found that all callers of free_pcppages_bulk() currently
> > > update pcp->count immediately after so it's natural to do it inside
> > > free_pcppages_bulk().
> > > 
> > > No functionality or performance change is expected from this patch.
> > > 
> > > Suggested-by: Matthew Wilcox <willy@infradead.org>
> > > Signed-off-by: Aaron Lu <aaron.lu@intel.com>
> > > ---
> > >  mm/page_alloc.c | 10 +++-------
> > >  1 file changed, 3 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > index cb416723538f..3154859cccd6 100644
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -1117,6 +1117,7 @@ static void free_pcppages_bulk(struct zone *zone, int count,
> > >  	int batch_free = 0;
> > >  	bool isolated_pageblocks;
> > >  
> > > +	pcp->count -= count;
> > >  	spin_lock(&zone->lock);
> > >  	isolated_pageblocks = has_isolate_pageblock(zone);
> > >  
> > 
> > Why modify pcp->count before the pages have actually been freed?
> 
> When count is still count and not zero after pages have actually been
> freed :-)
> 
> > 
> > I doubt that it matters too much, but at least /proc/zoneinfo uses 
> > zone->lock.  I think it should be done after the lock is dropped.
> 
> Agree that it looks a bit weird to do it beforehand and I just want to
> avoid adding one more local variable here.
> 
> pcp->count is not protected by zone->lock though so even we do it after
> dropping the lock, it could still happen that zoneinfo shows a wrong
> value of pcp->count while it should be zero(this isn't a problem since
> zoneinfo doesn't need to be precise).
> 
> Anyway, I'll follow your suggestion here to avoid confusion.

What about this: update pcp->count when page is dropped off pcp list.

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index cb416723538f..faa33eac1635 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1148,6 +1148,7 @@ static void free_pcppages_bulk(struct zone *zone, int count,
 			page = list_last_entry(list, struct page, lru);
 			/* must delete as __free_one_page list manipulates */
 			list_del(&page->lru);
+			pcp->count--;
 
 			mt = get_pcppage_migratetype(page);
 			/* MIGRATE_ISOLATE page should not go to pcplists */
@@ -2416,10 +2417,8 @@ void drain_zone_pages(struct zone *zone, struct per_cpu_pages *pcp)
 	local_irq_save(flags);
 	batch = READ_ONCE(pcp->batch);
 	to_drain = min(pcp->count, batch);
-	if (to_drain > 0) {
+	if (to_drain > 0)
 		free_pcppages_bulk(zone, to_drain, pcp);
-		pcp->count -= to_drain;
-	}
 	local_irq_restore(flags);
 }
 #endif
@@ -2441,10 +2440,8 @@ static void drain_pages_zone(unsigned int cpu, struct zone *zone)
 	pset = per_cpu_ptr(zone->pageset, cpu);
 
 	pcp = &pset->pcp;
-	if (pcp->count) {
+	if (pcp->count)
 		free_pcppages_bulk(zone, pcp->count, pcp);
-		pcp->count = 0;
-	}
 	local_irq_restore(flags);
 }
 
@@ -2668,7 +2665,6 @@ static void free_unref_page_commit(struct page *page, unsigned long pfn)
 	if (pcp->count >= pcp->high) {
 		unsigned long batch = READ_ONCE(pcp->batch);
 		free_pcppages_bulk(zone, batch, pcp);
-		pcp->count -= batch;
 	}
 }