Implement private fallback if immediate allocation from mempool fails. Standard mempool_alloc() fallback can yield a deadlock when only the calling process is able to refill the pool. In out-of-memory situations, instead of waiting for itself, kmirrord now waits for someone else to free some space, using a standard blocking allocation. Signed-off-by: Daniel Kobras <kobras@xxxxxxxx> --- [long story] Hi! On an nForce4-equipped machine with two SATA disk in raid1 setup using dmraid, we experienced frequent deadlock of the system under high i/o load. 'cat /dev/zero > ~/zero' was the most reliable way to reproduce them: Randomly after a few GB, 'cp' would be left in 'D' state along with kjournald and kmirrord. The functions cp and kjournald were blocked in did vary, but kmirrord's wchan always pointed to 'mempool_alloc()'. We've seen this pattern on 2.6.15 and 2.6.17 kernels. http://lkml.org/lkml/2005/4/20/142 indicates that this problem has been around even before. So much for the facts, here's my interpretation: mempool_alloc() first tries to atomically allocate the requested memory, or falls back to hand out preallocated chunks from the mempool. If both fail, it puts the calling process (kmirrord in this case) on a private waitqueue until somebody refills the pool. Where the only 'somebody' is kmirrord itself, so we have a deadlock. I worked around this problem by falling back to a (blocking) kmalloc when before kmirrord would have ended up on the waitqueue. This defeats part of the benefits of using the mempool, but at least keeps the system running. And it could be done with a two-line change. Note that mempool_alloc() clears the GFP_NOIO flag internally, and only uses it to decide whether to wait or return an error if immediate allocation fails, so the attached patch doesn't change behaviour in the non-deadlocking case. Path is against current git (2.6.18-rc4), but should apply to earlier versions as well. I've tested on 2.6.15, where this patch makes the difference between random lockup and a stable system. Regards, Daniel. diff -r dcc321d1340a -r d52bb3a14d60 drivers/md/dm-raid1.c --- a/drivers/md/dm-raid1.c Sun Aug 06 19:00:05 2006 +0000 +++ b/drivers/md/dm-raid1.c Mon Aug 07 23:16:44 2006 +0200 @@ -255,7 +255,9 @@ static struct region *__rh_alloc(struct struct region *reg, *nreg; read_unlock(&rh->hash_lock); - nreg = mempool_alloc(rh->region_pool, GFP_NOIO); + nreg = mempool_alloc(rh->region_pool, GFP_ATOMIC); + if (unlikely(!nreg)) + nreg = kmalloc(sizeof(struct region), GFP_NOIO); nreg->state = rh->log->type->in_sync(rh->log, region, 1) ? RH_CLEAN : RH_NOSYNC; nreg->rh = rh; -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
