Tetsuo Handa wrote:
> Konrad Rzeszutek Wilk wrote:
> > On Sat, May 24, 2014 at 11:22:09PM +0900, Tetsuo Handa wrote:
> > > Hello.
> > >
> > > I tried to test whether it is OK (from point of view of reentrant) to use
> > > mutex_lock() or mutex_lock_killable() inside shrinker functions when shrinker
> > > functions do memory allocation, for drivers/gpu/drm/ttm/ttm_page_alloc_dma.c is
> > > doing memory allocation with mutex lock held inside ttm_dma_pool_shrink_scan().
> > >
> > > If I compile a test module shown below which mimics extreme case of what
> > > ttm_dma_pool_shrink_scan() will do
> >
> > And ttm_pool_shrink_scan.
>
> I don't know why but ttm_pool_shrink_scan() does not take mutex.
>
Well, it seems to me that ttm_pool_shrink_scan() not taking mutex is a bug
which could lead to stack overflow if kmalloc() in ttm_page_pool_free()
triggered recursion.
shrink_slab()
=> ttm_pool_shrink_scan()
=> ttm_page_pool_free()
=> kmalloc(GFP_KERNEL)
=> shrink_slab()
=> ttm_pool_shrink_scan()
=> ttm_page_pool_free()
=> kmalloc(GFP_KERNEL)
Maybe shrink_slab() should be updated not to call same shrinker in parallel?
Also, it seems to me that ttm_dma_pool_shrink_scan() has potential division
by 0 bug as described below. Is this patch correct?
----------
>From 4a65744a300e14e5e202c5f13ba2759e1e797d29 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Date: Thu, 29 May 2014 18:25:42 +0900
Subject: [PATCH] gpu/drm/ttm: Use mutex_trylock() for shrinker functions.
I can observe that RHEL7 environment stalls with 100% CPU usage when a
certain type of memory pressure is given. While the shrinker functions
are called by shrink_slab() before the OOM killer is triggered, the stall
lasts for many minutes.
One of reasons of this stall is that
ttm_dma_pool_shrink_count()/ttm_dma_pool_shrink_scan() are called and
are blocked at mutex_lock(&_manager->lock). GFP_KERNEL allocation with
_manager->lock held causes someone (including kswapd) to deadlock when
these functions are called due to memory pressure. This patch changes
"mutex_lock();" to "if (!mutex_trylock()) return ...;" in order to
avoid deadlock.
At the same time, this patch fixes potential division by 0 due to
unconditionally doing "% _manager->npools". This is because
list_empty(&_manager->pools) being false does not guarantee that
_manager->npools != 0 after taking the _manager->lock because
_manager->npools is updated under the _manager->lock.
At the same time, this patch moves updating of start_pool variable
in order to avoid skipping when choosing a pool to shrink in
round-robin style. The start_pool is changed from "atomic_t" to
"unsigned int" because it is now updated under the _manager->lock.
Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Cc: stable <stable@xxxxxxxxxx> [3.3+]
---
drivers/gpu/drm/ttm/ttm_page_alloc_dma.c | 15 ++++++++++-----
1 file changed, 10 insertions(+), 5 deletions(-)
diff --git a/drivers/gpu/drm/ttm/ttm_page_alloc_dma.c b/drivers/gpu/drm/ttm/ttm_page_alloc_dma.c
index fb8259f..5e332b4 100644
--- a/drivers/gpu/drm/ttm/ttm_page_alloc_dma.c
+++ b/drivers/gpu/drm/ttm/ttm_page_alloc_dma.c
@@ -1004,9 +1004,9 @@ EXPORT_SYMBOL_GPL(ttm_dma_unpopulate);
static unsigned long
ttm_dma_pool_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
{
- static atomic_t start_pool = ATOMIC_INIT(0);
+ static unsigned int start_pool;
unsigned idx = 0;
- unsigned pool_offset = atomic_add_return(1, &start_pool);
+ unsigned pool_offset;
unsigned shrink_pages = sc->nr_to_scan;
struct device_pools *p;
unsigned long freed = 0;
@@ -1014,8 +1014,11 @@ ttm_dma_pool_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
if (list_empty(&_manager->pools))
return SHRINK_STOP;
- mutex_lock(&_manager->lock);
- pool_offset = pool_offset % _manager->npools;
+ if (!mutex_trylock(&_manager->lock))
+ return SHRINK_STOP;
+ if (!_manager->npools)
+ goto out;
+ pool_offset = ++start_pool % _manager->npools;
list_for_each_entry(p, &_manager->pools, pools) {
unsigned nr_free;
@@ -1034,6 +1037,7 @@ ttm_dma_pool_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
p->pool->dev_name, p->pool->name, current->pid,
nr_free, shrink_pages);
}
+out:
mutex_unlock(&_manager->lock);
return freed;
}
@@ -1044,7 +1048,8 @@ ttm_dma_pool_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
struct device_pools *p;
unsigned long count = 0;
- mutex_lock(&_manager->lock);
+ if (!mutex_trylock(&_manager->lock))
+ return 0;
list_for_each_entry(p, &_manager->pools, pools)
count += p->pool->npages_free;
mutex_unlock(&_manager->lock);
--
1.8.3.1
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