Re: System freezes after OOM

From: Ondrej Kozina
Date: Thu Jul 14 2016 - 11:23:14 EST

On 07/14/2016 02:51 PM, Michal Hocko wrote:
On Wed 13-07-16 11:02:15, Mikulas Patocka wrote:
On Wed, 13 Jul 2016, Michal Hocko wrote:

We are discussing several topics together so let's focus on this
particlar thing for now

The kernel 4.7-rc almost deadlocks in another way. The machine got stuck
and the following stacktrace was obtained when swapping to dm-crypt.

We can see that dm-crypt does a mempool allocation. But the mempool
allocation somehow falls into throttle_vm_writeout. There, it waits for
0.1 seconds. So, as a result, the dm-crypt worker thread ends up
processing requests at an unusually slow rate of 10 requests per second
and it results in the machine being stuck (it would proabably recover if
we waited for extreme amount of time).

Hmm, that throttling is there since ever basically. I do not see what
would have changed that recently, but I haven't looked too close to be

I agree that throttling a flusher (which this worker definitely is)
doesn't look like a correct thing to do. We have PF_LESS_THROTTLE for
this kind of things. So maybe the right thing to do is to use this flag
for the dm_crypt worker:

diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
index 4f3cb3554944..0b806810efab 100644
--- a/drivers/md/dm-crypt.c
+++ b/drivers/md/dm-crypt.c
@@ -1392,11 +1392,14 @@ static void kcryptd_async_done(struct crypto_async_request *async_req,
static void kcryptd_crypt(struct work_struct *work)
struct dm_crypt_io *io = container_of(work, struct dm_crypt_io, work);
+ unsigned int pflags = current->flags;

+ current->flags |= PF_LESS_THROTTLE;
if (bio_data_dir(io->base_bio) == READ)
+ tsk_restore_flags(current, pflags, PF_LESS_THROTTLE);

static void kcryptd_queue_crypt(struct dm_crypt_io *io)

^^^ That fixes just one specific case - but there may be other threads
doing mempool allocations in the device mapper subsystem - and you would
need to mark all of them.

Now that I am thinking about it some more. Are there any mempool users
which would actually want to be throttled? I would expect mempool users
are necessary to push IO through and throttle them sounds like a bad
decision in the first place but there might be other mempool users which
could cause issues. Anyway how about setting PF_LESS_THROTTLE
unconditionally inside mempool_alloc? Something like the following:

diff --git a/mm/mempool.c b/mm/mempool.c
index 8f65464da5de..e21fb632983f 100644
--- a/mm/mempool.c
+++ b/mm/mempool.c
@@ -310,7 +310,8 @@ EXPORT_SYMBOL(mempool_resize);
void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)
- void *element;
+ unsigned int pflags = current->flags;
+ void *element = NULL;
unsigned long flags;
wait_queue_t wait;
gfp_t gfp_temp;
@@ -327,6 +328,12 @@ void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)

gfp_temp = gfp_mask & ~(__GFP_DIRECT_RECLAIM|__GFP_IO);

+ /*
+ * Make sure that the allocation doesn't get throttled during the
+ * reclaim
+ */
+ if (gfpflags_allow_blocking(gfp_mask))
+ current->flags |= PF_LESS_THROTTLE;
if (likely(pool->curr_nr)) {
@@ -339,7 +346,7 @@ void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)

element = pool->alloc(gfp_temp, pool->pool_data);
if (likely(element != NULL))
- return element;
+ goto out;

spin_lock_irqsave(&pool->lock, flags);
if (likely(pool->curr_nr)) {
@@ -352,7 +359,7 @@ void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)
* for debugging.
- return element;
+ goto out;

@@ -369,7 +376,7 @@ void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)
/* We must not sleep if !__GFP_DIRECT_RECLAIM */
if (!(gfp_mask & __GFP_DIRECT_RECLAIM)) {
spin_unlock_irqrestore(&pool->lock, flags);
- return NULL;
+ goto out;

/* Let's wait for someone else to return an element to @pool */
@@ -386,6 +393,10 @@ void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)

finish_wait(&pool->wait, &wait);
goto repeat_alloc;
+ if (gfpflags_allow_blocking(gfp_mask))
+ tsk_restore_flags(current, pflags, PF_LESS_THROTTLE);
+ return element;

As Mikulas pointed out, this doesn't work. The system froze as well with the patch above. Will try to tweak the patch with Mikulas's suggestion...