Re: [PATCH 2/6] mm/page_alloc: Disassociate the pcp->high from pcp->batch

Mel Gorman
2021-05-27T10:52:48Z

On Wed, May 26, 2021 at 08:14:13PM +0200, Vlastimil Babka wrote:
> > @@ -6698,11 +6717,10 @@ static void __zone_set_pageset_high_and_batch(struct zone *zone, unsigned long h
> > */
> > static void zone_set_pageset_high_and_batch(struct zone *zone)
> > {
> > - unsigned long new_high, new_batch;
> > + int new_high, new_batch;
> >
> > - new_batch = zone_batchsize(zone);
> > - new_high = 6 * new_batch;
> > - new_batch = max(1UL, 1 * new_batch);
> > + new_batch = max(1, zone_batchsize(zone));
> > + new_high = zone_highsize(zone, new_batch);
> >
> > if (zone->pageset_high == new_high &&
> > zone->pageset_batch == new_batch)
> > @@ -8170,6 +8188,12 @@ static void __setup_per_zone_wmarks(void)
> > zone->_watermark[WMARK_LOW] = min_wmark_pages(zone) + tmp;
> > zone->_watermark[WMARK_HIGH] = min_wmark_pages(zone) + tmp * 2;
> >
> > + /*
> > + * The watermark size have changed so update the pcpu batch
> > + * and high limits or the limits may be inappropriate.
> > + */
> > + zone_set_pageset_high_and_batch(zone);
>
> Hm so this puts the call in the path of various watermark related sysctl
> handlers, but it's not protected by pcp_batch_high_lock. The zone lock won't
> help against zone_pcp_update() from a hotplug handler. On the other hand, since
> hotplug handlers also call __setup_per_zone_wmarks(), the zone_pcp_update()
> calls there are now redundant and could be removed, no?
> But later there will be a new sysctl in patch 6/6 using pcp_batch_high_lock,
> thus that one will not be protected against the watermark related sysctl
> handlers that reach here.
>
> To solve all this, seems like the static lock in setup_per_zone_wmarks() could
> become a top-level visible lock and pcp high/batch updates could switch to that
> one instead of own pcp_batch_high_lock. And zone_pcp_update() calls from hotplug
> handlers could be removed.
>

Hmm, the locking has very different hold times. The static lock in
setup_per_zone_wmarks is a spinlock that protects against parallel updates
of watermarks and is held for a short duration. The pcp_batch_high_lock
is a mutex that is held for a relatively long time while memory is being
offlined and can sleep. Memory hotplug updates the watermarks without
pcp_batch_high_lock held so overall, unifying the locking there should
be a separate series.

How about this as a fix for this patch?

---8<---
mm/page_alloc: Disassociate the pcp->high from pcp->batch -fix

Vlastimil Babka noted that __setup_per_zone_wmarks updating pcp->high
did not protect watermark-related sysctl handlers from a parallel
memory hotplug operations. This patch moves the PCP update to
setup_per_zone_wmarks and updates the PCP high value while protected
by the pcp_batch_high_lock mutex.

This is a fix to the mmotm patch mm-page_alloc-disassociate-the-pcp-high-from-pcp-batch.patch.
It'll cause a conflict with mm-page_alloc-adjust-pcp-high-after-cpu-hotplug-events.patch
but the resolution is simply to change the caller in setup_per_zone_wmarks
to zone_pcp_update(zone, 0)

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
mm/page_alloc.c | 14 ++++++++------
1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 329b71e41db4..b1b3c66e9d88 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -8199,12 +8199,6 @@ static void __setup_per_zone_wmarks(void)
zone->_watermark[WMARK_LOW] = min_wmark_pages(zone) + tmp;
zone->_watermark[WMARK_HIGH] = min_wmark_pages(zone) + tmp * 2;

- /*
- * The watermark size have changed so update the pcpu batch
- * and high limits or the limits may be inappropriate.
- */
- zone_set_pageset_high_and_batch(zone);
-
spin_unlock_irqrestore(&zone->lock, flags);
}

@@ -8221,11 +8215,19 @@ static void __setup_per_zone_wmarks(void)
*/
void setup_per_zone_wmarks(void)
{
+ struct zone *zone;
static DEFINE_SPINLOCK(lock);

spin_lock(&lock);
__setup_per_zone_wmarks();
spin_unlock(&lock);
+
+ /*
+ * The watermark size have changed so update the pcpu batch
+ * and high limits or the limits may be inappropriate.
+ */
+ for_each_zone(zone)
+ zone_pcp_update(zone);
}

/*