Hi The GFP_NOIO allocation frees clean cached pages. The GFP_NOWAIT allocation doesn't. Your patch would incorrectly reuse buffers in a situation when the memory is filled with clean cached pages. Here I'm proposing an alternate patch that first tries GFP_NOWAIT allocation, then drops the lock and tries GFP_NOIO allocation. Note that the root cause why you are seeing this stacktrace is, that your block device is congested - i.e. there are too many requests in the device's queue - and note that fixing this wait won't fix the root cause (congested device). The congestion limits are set in blk_queue_congestion_threshold to 7/8 to 13/16 size of the nr_requests value. If you don't want your device to report the congested status, you can increase /sys/block/<device>/queue/nr_requests - you should test if your chromebook is faster of slower with this setting increased. But note that this setting won't increase the IO-per-second of the device. Mikulas On Thu, 17 Nov 2016, Douglas Anderson wrote: > We've seen in-field reports showing _lots_ (18 in one case, 41 in > another) of tasks all sitting there blocked on: > > mutex_lock+0x4c/0x68 > dm_bufio_shrink_count+0x38/0x78 > shrink_slab.part.54.constprop.65+0x100/0x464 > shrink_zone+0xa8/0x198 > > In the two cases analyzed, we see one task that looks like this: > > Workqueue: kverityd verity_prefetch_io > > __switch_to+0x9c/0xa8 > __schedule+0x440/0x6d8 > schedule+0x94/0xb4 > schedule_timeout+0x204/0x27c > schedule_timeout_uninterruptible+0x44/0x50 > wait_iff_congested+0x9c/0x1f0 > shrink_inactive_list+0x3a0/0x4cc > shrink_lruvec+0x418/0x5cc > shrink_zone+0x88/0x198 > try_to_free_pages+0x51c/0x588 > __alloc_pages_nodemask+0x648/0xa88 > __get_free_pages+0x34/0x7c > alloc_buffer+0xa4/0x144 > __bufio_new+0x84/0x278 > dm_bufio_prefetch+0x9c/0x154 > verity_prefetch_io+0xe8/0x10c > process_one_work+0x240/0x424 > worker_thread+0x2fc/0x424 > kthread+0x10c/0x114 > > ...and that looks to be the one holding the mutex. > > The problem has been reproduced on fairly easily: > 0. Be running Chrome OS w/ verity enabled on the root filesystem > 1. Pick test patch: http://crosreview.com/412360 > 2. Install launchBalloons.sh and balloon.arm from > http://crbug.com/468342 > ...that's just a memory stress test app. > 3. On a 4GB rk3399 machine, run > nice ./launchBalloons.sh 4 900 100000 > ...that tries to eat 4 * 900 MB of memory and keep accessing. > 4. Login to the Chrome web browser and restore many tabs > > With that, I've seen printouts like: > DOUG: long bufio 90758 ms > ...and stack trace always show's we're in dm_bufio_prefetch(). > > The problem is that we try to allocate memory with GFP_NOIO while > we're holding the dm_bufio lock. Instead we should be using > GFP_NOWAIT. Using GFP_NOIO can cause us to sleep while holding the > lock and that causes the above problems. > > The current behavior explained by David Rientjes: > > It will still try reclaim initially because __GFP_WAIT (or > __GFP_KSWAPD_RECLAIM) is set by GFP_NOIO. This is the cause of > contention on dm_bufio_lock() that the thread holds. You want to > pass GFP_NOWAIT instead of GFP_NOIO to alloc_buffer() when holding a > mutex that can be contended by a concurrent slab shrinker (if > count_objects didn't use a trylock, this pattern would trivially > deadlock). > > Suggested-by: David Rientjes <rientjes@xxxxxxxxxx> > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx> > --- > Note that this change was developed and tested against the Chrome OS > 4.4 kernel tree, not mainline. Due to slight differences in verity > between mainline and Chrome OS it became too difficult to reproduce my > testing setup on mainline. This patch still seems correct and > relevant to upstream, so I'm posting it. If this is not acceptible to > you then please ignore this patch. > > Also note that when I tested the Chrome OS 3.14 kernel tree I couldn't > reproduce the long delays described in the patch. Presumably > something changed in either the kernel config or the memory management > code between the two kernel versions that made this crop up. In a > similar vein, it is possible that problems described in this patch are > no longer reproducible upstream. However, the arguments made in this > patch (that we don't want to block while holding the mutex) still > apply so I think the patch may still have merit. > > drivers/md/dm-bufio.c | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c > index b3ba142e59a4..3c767399cc59 100644 > --- a/drivers/md/dm-bufio.c > +++ b/drivers/md/dm-bufio.c > @@ -827,7 +827,8 @@ static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client > * dm-bufio is resistant to allocation failures (it just keeps > * one buffer reserved in cases all the allocations fail). > * So set flags to not try too hard: > - * GFP_NOIO: don't recurse into the I/O layer > + * GFP_NOWAIT: don't wait; if we need to sleep we'll release our > + * mutex and wait ourselves. > * __GFP_NORETRY: don't retry and rather return failure > * __GFP_NOMEMALLOC: don't use emergency reserves > * __GFP_NOWARN: don't print a warning in case of failure > @@ -837,7 +838,8 @@ static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client > */ > while (1) { > if (dm_bufio_cache_size_latch != 1) { > - b = alloc_buffer(c, GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN); > + b = alloc_buffer(c, GFP_NOWAIT | __GFP_NORETRY | > + __GFP_NOMEMALLOC | __GFP_NOWARN); > if (b) > return b; > } > -- > 2.8.0.rc3.226.g39d4020 > > -- > dm-devel mailing list > dm-devel@xxxxxxxxxx > https://www.redhat.com/mailman/listinfo/dm-devel > From: Mikulas Patocka <mpatocka@xxxxxxxxxx> Subject: dm-bufio: drop the lock when doing GFP_NOIO alloaction Drop the lock when doing GFP_NOIO alloaction beacuse the allocation can take some time. Note that we won't do GFP_NOIO allocation when we loop for the second time, because the lock shouldn't be dropped between __wait_for_free_buffer and __get_unclaimed_buffer. Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx> --- drivers/md/dm-bufio.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) Index: linux-2.6/drivers/md/dm-bufio.c =================================================================== --- linux-2.6.orig/drivers/md/dm-bufio.c +++ linux-2.6/drivers/md/dm-bufio.c @@ -822,11 +822,13 @@ enum new_flag { static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client *c, enum new_flag nf) { struct dm_buffer *b; + bool tried_noio_alloc = false; /* * dm-bufio is resistant to allocation failures (it just keeps * one buffer reserved in cases all the allocations fail). * So set flags to not try too hard: + * GFP_NOWAIT: don't sleep and don't release cache * GFP_NOIO: don't recurse into the I/O layer * __GFP_NORETRY: don't retry and rather return failure * __GFP_NOMEMALLOC: don't use emergency reserves @@ -837,7 +839,7 @@ static struct dm_buffer *__alloc_buffer_ */ while (1) { if (dm_bufio_cache_size_latch != 1) { - b = alloc_buffer(c, GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN); + b = alloc_buffer(c, GFP_NOWAIT | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN); if (b) return b; } @@ -845,6 +847,15 @@ static struct dm_buffer *__alloc_buffer_ if (nf == NF_PREFETCH) return NULL; + if (dm_bufio_cache_size_latch != 1 && !tried_noio_alloc) { + dm_bufio_unlock(c); + b = alloc_buffer(c, GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN); + dm_bufio_lock(c); + if (b) + return b; + tried_noio_alloc = true; + } + if (!list_empty(&c->reserved_buffers)) { b = list_entry(c->reserved_buffers.next, struct dm_buffer, lru_list); -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel