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vmscan: shrinker->nr updates race and go wrong

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shrink_slab() allows shrinkers to be called in parallel so the
struct shrinker can be updated concurrently. It does not provide any
exclusio for such updates, so we can get the shrinker->nr value
increasing or decreasing incorrectly.

As a result, when a shrinker repeatedly returns a value of -1 (e.g.
a VFS shrinker called w/ GFP_NOFS), the shrinker->nr goes haywire,
sometimes updating with the scan count that wasn't used, sometimes
losing it altogether. Worse is when a shrinker does work and that
update is lost due to racy updates, which means the shrinker will do
the work again!

Fix this by making the total_scan calculations independent of
shrinker->nr, and making the shrinker->nr updates atomic w.r.t. to
other updates via cmpxchg loops.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Dave Chinner 2011-07-08 14:14:35 +10:00 committed by Al Viro
parent 095760730c
commit acf92b485c
1 changed files with 32 additions and 13 deletions
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45
mm/vmscan.c
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@ -251,17 +251,29 @@ unsigned long shrink_slab(struct shrink_control *shrink,
unsigned long total_scan;
unsigned long max_pass;
int shrink_ret = 0;
long nr;
long new_nr;
/*
* copy the current shrinker scan count into a local variable
* and zero it so that other concurrent shrinker invocations
* don't also do this scanning work.
*/
do {
nr = shrinker->nr;
} 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);
shrinker->nr += delta;
if (shrinker->nr < 0) {
total_scan += delta;
if (total_scan < 0) {
printk(KERN_ERR "shrink_slab: %pF negative objects to "
"delete nr=%ld\n",
shrinker->shrink, shrinker->nr);
shrinker->nr = max_pass;
shrinker->shrink, total_scan);
total_scan = max_pass;
}
/*
@ -269,13 +281,10 @@ unsigned long shrink_slab(struct shrink_control *shrink,
* never try to free more than twice the estimate number of
* freeable entries.
*/
if (shrinker->nr > max_pass * 2)
shrinker->nr = max_pass * 2;
if (total_scan > max_pass * 2)
total_scan = max_pass * 2;
total_scan = shrinker->nr;
shrinker->nr = 0;
trace_mm_shrink_slab_start(shrinker, shrink, total_scan,
trace_mm_shrink_slab_start(shrinker, shrink, nr,
nr_pages_scanned, lru_pages,
max_pass, delta, total_scan);
@ -296,9 +305,19 @@ unsigned long shrink_slab(struct shrink_control *shrink,
cond_resched();
}
shrinker->nr += total_scan;
trace_mm_shrink_slab_end(shrinker, shrink_ret, total_scan,
shrinker->nr);
/*
* move the unused scan count back into the shrinker in a
* manner that handles concurrent updates. If we exhausted the
* scan, there is no need to do an update.
*/
do {
nr = shrinker->nr;
new_nr = total_scan + nr;
if (total_scan <= 0)
break;
} while (cmpxchg(&shrinker->nr, nr, new_nr) != nr);
trace_mm_shrink_slab_end(shrinker, shrink_ret, nr, new_nr);
}
up_read(&shrinker_rwsem);
out: