Re: [ 13/40] vmscan: shrinker->nr updates race and go wrong

[Mel Gorman <mgorman@suse.de>]

On Sun, Jul 29, 2012 at 09:29:30PM +0100, Ben Hutchings wrote:
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -251,17 +251,29 @@ unsigned long shrink_slab(struct shrink_
> >  		unsigned long total_scan;
> [...]
> > +		total_scan += delta;
> > +		if (total_scan < 0) {
> [...]
> 
> This condition is never true since total_scan is unsigned.
> I think 3.0.y needs this as well:
> 
> commit 635697c663f38106063d5659f0cf2e45afcd4bb5
> Author: Konstantin Khlebnikov <khlebnikov@openvz.org>
> Date:   Thu Dec 8 14:33:51 2011 -0800
> 
>     vmscan: fix initial shrinker size handling
> 

You're right, thanks for pointing it out as I missed it. Greg, this commit
is already in 3.2 but can you pick it up for 3.0-stable please?

---8<---
vmscan: fix initial shrinker size handling

commit 635697c663f38106063d5659f0cf2e45afcd4bb5 upstream.

Stable note: The commit [acf92b48: vmscan: shrinker->nr updates race and
	go wrong] aimed to reduce excessive reclaim of slab objects but
	had bug in how it treated shrinker functions that returned -1.

A shrinker function can return -1, means that it cannot do anything
without a risk of deadlock.  For example prune_super() does this if it
cannot grab a superblock refrence, even if nr_to_scan=0.  Currently we
interpret this -1 as a ULONG_MAX size shrinker and evaluate `total_scan'
according to this.  So the next time around this shrinker can cause
really big pressure.  Let's skip such shrinkers instead.

Also make total_scan signed, otherwise the check (total_scan < 0) below
never works.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 mm/vmscan.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index e0afff3..bd8a540 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -248,12 +248,16 @@ unsigned long shrink_slab(struct shrink_control *shrink,
 
 	list_for_each_entry(shrinker, &shrinker_list, list) {
 		unsigned long long delta;
-		unsigned long total_scan;
-		unsigned long max_pass;
+		long total_scan;
+		long max_pass;
 		int shrink_ret = 0;
 		long nr;
 		long new_nr;
 
+		max_pass = do_shrinker_shrink(shrinker, shrink, 0);
+		if (max_pass <= 0)
+			continue;
+
 		/*
 		 * copy the current shrinker scan count into a local variable
 		 * and zero it so that other concurrent shrinker invocations
@@ -264,7 +268,6 @@ unsigned long shrink_slab(struct shrink_control *shrink,
 		} while (cmpxchg(&shrinker->nr, nr, 0) != nr);
 
 		total_scan = nr;
-		max_pass = do_shrinker_shrink(shrinker, shrink, 0);
 		delta = (4 * nr_pages_scanned) / shrinker->seeks;
 		delta *= max_pass;
 		do_div(delta, lru_pages + 1);