Patchwork [v2,1/1] block: fix blk_queue_split() resource exhaustion

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Submitter Mikulas Patocka
Date Jan. 6, 2017, 5:34 p.m.
Message ID <alpine.LRH.2.02.1701061151010.13944@file01.intranet.prod.int.rdu2.redhat.com>
Download mbox | patch
Permalink /patch/9501543/
State Superseded, archived
Delegated to: Mike Snitzer
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Comments

Mikulas Patocka - Jan. 6, 2017, 5:34 p.m.
On Fri, 6 Jan 2017, Mikulas Patocka wrote:

> 
> 
> On Wed, 4 Jan 2017, Mike Snitzer wrote:
> 
> > On Wed, Jan 04 2017 at 12:12am -0500,
> > NeilBrown <neilb@suse.com> wrote:
> > 
> > > > Suggested-by: NeilBrown <neilb@suse.com>
> > > > Signed-off-by: Jack Wang <jinpu.wang@profitbricks.com>
> > > > ---
> > > >  block/blk-core.c | 20 ++++++++++++++++++++
> > > >  1 file changed, 20 insertions(+)
> > > >
> > > > diff --git a/block/blk-core.c b/block/blk-core.c
> > > > index 9e3ac56..47ef373 100644
> > > > --- a/block/blk-core.c
> > > > +++ b/block/blk-core.c
> > > > @@ -2138,10 +2138,30 @@ blk_qc_t generic_make_request(struct bio *bio)
> > > >  		struct request_queue *q = bdev_get_queue(bio->bi_bdev);
> > > >  
> > > >  		if (likely(blk_queue_enter(q, __GFP_DIRECT_RECLAIM) == 0)) {
> > > > +			struct bio_list lower, same, hold;
> > > > +
> > > > +			/* Create a fresh bio_list for all subordinate requests */
> > > > +			bio_list_init(&hold);
> > > > +			bio_list_merge(&hold, &bio_list_on_stack);
> > > > +			bio_list_init(&bio_list_on_stack);
> > > >  
> > > >  			ret = q->make_request_fn(q, bio);
> > > >  
> > > >  			blk_queue_exit(q);
> > > > +			/* sort new bios into those for a lower level
> > > > +			 * and those for the same level
> > > > +			 */
> > > > +			bio_list_init(&lower);
> > > > +			bio_list_init(&same);
> > > > +			while ((bio = bio_list_pop(&bio_list_on_stack)) != NULL)
> > > > +				if (q == bdev_get_queue(bio->bi_bdev))
> > > > +					bio_list_add(&same, bio);
> > > > +				else
> > > > +					bio_list_add(&lower, bio);
> > > > +			/* now assemble so we handle the lowest level first */
> > > > +			bio_list_merge(&bio_list_on_stack, &lower);
> > > > +			bio_list_merge(&bio_list_on_stack, &same);
> > > > +			bio_list_merge(&bio_list_on_stack, &hold);
> > > >  
> > > >  			bio = bio_list_pop(current->bio_list);
> > > >  		} else {
> > > > -- 
> > > > 2.7.4
> > 
> > Mikulas, would you be willing to try the below patch with the
> > dm-snapshot deadlock scenario and report back on whether it fixes that?
> > 
> > Patch below looks to be the same as here:
> > https://marc.info/?l=linux-raid&m=148232453107685&q=p3
> > 
> > Neil and/or others if that isn't the patch that should be tested please
> > provide a pointer to the latest.
> > 
> > Thanks,
> > Mike
> 
> The bad news is that this doesn't fix the snapshot deadlock.
> 
> I created a test program for the snapshot deadlock bug (it was originally 
> created years ago to test for a different bug, so it contains some cruft). 
> You also need to insert "if (ci->sector_count) msleep(100);" to the end of 
> __split_and_process_non_flush to make the kernel sleep when splitting the 
> bio.
> 
> And with the above above patch, the snapshot deadlock bug still happens.
> 
> Mikulas
> 
> 
> #define _XOPEN_SOURCE 500
> #define _GNU_SOURCE
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
> #include <fcntl.h>
> #include <string.h>
> #include <errno.h>
> #include <malloc.h>
> #include <pthread.h>
> #include <asm/unistd.h>
> 
> /*
>  * Change "VG" symbol to a volume group name that you are using.
>  *
>  * You must apply this patch to the kernel to trigger the bug:
>  * Index: linux-4.10-rc2/drivers/md/dm.c
>  * ===================================================================
>  * --- linux-4.10-rc2.orig/drivers/md/dm.c
>  * +++ linux-4.10-rc2/drivers/md/dm.c
>  * @@ -1223,6 +1223,9 @@ static int __split_and_process_non_flush
>  *         ci->sector += len;
>  *         ci->sector_count -= len;
>  * 
>  * +       if (ci->sector_count)
>  * +               msleep(100);
>  * +
>  *         return 0;
>  *  }
>  * 
>  */
> 
> #define VG		"vg1"
> #define LV		"test_lv"
> #define LV_SNAP		"test_snap"
> #define MEGABYTES	"12"
> #define SNAP_MEGABYTES	"16"
> #define THREADS		1
> #define BS		4096
> #define SKEW		512
> #define ORIG_PATTERN	'p'
> #define NEW_PATTERN	'n'
> 
> enum {
> 	IOPRIO_CLASS_NONE,
> 	IOPRIO_CLASS_RT,
> 	IOPRIO_CLASS_BE,
> 	IOPRIO_CLASS_IDLE,
> };
> 
> enum {
> 	IOPRIO_WHO_PROCESS = 1,
> 	IOPRIO_WHO_PGRP,
> 	IOPRIO_WHO_USER,
> };
> 
> #define IOPRIO_CLASS_SHIFT	13
> 
> static inline int ioprio_set(int which, int who, int ioprio)
> {
> 	return syscall(__NR_ioprio_set, which, who, ioprio);
> }
> 
> static inline int ioprio_get(int which, int who)
> {
> 	return syscall(__NR_ioprio_get, which, who);
> }
> 
> #define PRIO_READER	((IOPRIO_CLASS_IDLE << IOPRIO_CLASS_SHIFT) | 0xff)
> #define PRIO_WRITER	(IOPRIO_CLASS_RT << IOPRIO_CLASS_SHIFT)
> 
> static void do_cmd(char *cmd, int ign_err)
> {
> 	int r;
> 	fprintf(stderr, "* %s\n", cmd);
> 	r = system(cmd);
> 	if (r) {
> 		if (r == -1) {
> 			perror("system");
> 		} else {
> 			if (ign_err) return;
> 			fprintf(stderr, "return code %x\n", r);
> 		}
> 		exit(1);
> 	}
> }
> 
> static char pattern[BS];
> 
> static int h_orig, h_snap;
> static int n;
> static long long test_of;
> static pthread_rwlock_t rw_lock_1;
> static pthread_rwlock_t rw_lock_2;
> static pthread_rwlock_t rw_lock_3;
> static volatile int started = 0;
> 
> static void pthread_error(int r)
> {
> 	fprintf(stderr, "pthread_error: %s\n", strerror(r));
> 	exit(1);
> }
> 
> static void *test_read(long long of)
> {
> 	int r;
> 	char *t = memalign(BS, BS);
> 	if (!t) perror("memalign"), exit(1);
> 	if ((r = pread(h_snap, t, BS, of)) != BS) {
> 		fprintf(stderr, "can't read (%d): %s\n", r, strerror(errno));
> 		exit(1);
> 	}
> 	if (memcmp(pattern, t, BS)) {
> 		int i;
> 		for (i = 0; i < BS; i++) if (t[i] != pattern[i]) break;
> 		fprintf(stderr, "!!!! SNAPSHOT VOLUME DAMAGE AT BLOCK OFFSET %llX, BYTE OFFSET %X: %02x != %02x\n", of, i, (unsigned char)t[i], (unsigned char)pattern[i]);
> 		exit(2);
> 	}
> 	free(t);
> 	return NULL;
> }
> 
> static void *test_thread(void *_)
> {
> 	int r;
> 	_ = _;
> 	//fprintf(stderr, "start\n");
> 	if ((r = ioprio_set(IOPRIO_WHO_PROCESS, 0, PRIO_READER))) perror("ioprio_set"), exit(1);
> 	if ((r = pthread_rwlock_rdlock(&rw_lock_2))) pthread_error(r);
> 	started = 1;
> 	if ((r = ioprio_get(IOPRIO_WHO_PROCESS, 0)) != PRIO_READER) {
> 		if (r == -1) perror("ioprio_get");
> 		else fprintf(stderr, "reader priority not set: %x\n", r);
> 		exit(1);
> 	}
> 	again:
> 	if ((r = pthread_rwlock_rdlock(&rw_lock_1))) pthread_error(r);
> 	if ((r = pthread_rwlock_unlock(&rw_lock_2))) pthread_error(r);
> 	if (test_of == -1) {
> 		if ((r = pthread_rwlock_unlock(&rw_lock_1))) pthread_error(r);
> 		//fprintf(stderr, "return\n");
> 		return NULL;
> 	}
> 	//fprintf(stderr, "test(%lld)\n", test_of);
> 	test_read(test_of);
> 	if ((r = pthread_rwlock_rdlock(&rw_lock_3))) pthread_error(r);
> 	if ((r = pthread_rwlock_unlock(&rw_lock_1))) pthread_error(r);
> 	if ((r = pthread_rwlock_rdlock(&rw_lock_2))) pthread_error(r);
> 	if ((r = pthread_rwlock_unlock(&rw_lock_3))) pthread_error(r);
> 	goto again;
> }
> 
> int main(void)
> {
> 	int i, j, r;
> 	char *np;
> 	pthread_t thr[THREADS];
> 
> 	memset(pattern, ORIG_PATTERN, sizeof pattern);
> 
> 	do_cmd("lvremove -f "VG"/"LV_SNAP"", 1);
> 	do_cmd("lvremove -f "VG"/"LV"", 1);
> 	do_cmd("lvcreate -L "MEGABYTES" -n "LV" "VG"", 0);
> 
> 	h_orig = open("/dev/mapper/"VG"-"LV"", O_RDWR);
> 	if (h_orig < 0) perror("open orig"), exit(1);
> 	if (lseek(h_orig, SKEW, SEEK_SET) == -1) perror("lseek"), exit(1);
> 	n = 0;
> 	while (write(h_orig, pattern, BS) == BS) {
> 		n++;
> 		fprintf(stderr, "creating %llx...\r", (long long)n * BS + SKEW);
> 	}
> 	if (fsync(h_orig)) perror("fsync"), exit(1);
> 	fprintf(stderr,"\n");
> 	lseek(h_orig, 0, SEEK_SET);
> 	close(h_orig);
> 
> 	do_cmd("lvcreate -L "SNAP_MEGABYTES" -n "LV_SNAP" -s "VG"/"LV"", 0);
> 
> 	h_orig = open("/dev/mapper/"VG"-"LV"", O_RDWR | O_DIRECT);
> 	if (h_orig < 0) perror("open orig"), exit(1);
> 
> 	h_snap = open("/dev/mapper/"VG"-"LV_SNAP"", O_RDONLY | O_DIRECT);
> 	if (h_snap < 0) perror("open snap"), exit(1);
> 
> 	if ((r = pthread_rwlock_init(&rw_lock_1, NULL))) pthread_error(r);
> 	if ((r = pthread_rwlock_init(&rw_lock_2, NULL))) pthread_error(r);
> 	if ((r = pthread_rwlock_init(&rw_lock_3, NULL))) pthread_error(r);
> 	if ((r = pthread_rwlock_wrlock(&rw_lock_1))) pthread_error(r);
> 	if ((r = pthread_rwlock_wrlock(&rw_lock_3))) pthread_error(r);
> 
> 	if ((r = ioprio_set(IOPRIO_WHO_PROCESS, 0, PRIO_WRITER))) perror("ioprio_set"), exit(1);
> 
> 	for (j = 0; j < THREADS; j++) {
> 		if ((r = pthread_create(&thr[j], NULL, test_thread, NULL))) pthread_error(r);
> 	}
> 	while (!started) usleep(1000);
> 
> 	if ((r = ioprio_get(IOPRIO_WHO_PROCESS, 0)) != PRIO_WRITER) {
> 		if (r == -1) perror("ioprio_get");
> 		else fprintf(stderr, "writer priority not set: %x\n", r);
> 		exit(1);
> 	}
> 
> 	np = memalign(BS, BS);
> 	if (!np) perror("memalign"), exit(1);
> 	memset(np, NEW_PATTERN, BS);
> 	for (i = 0; i < n; i++) {
> 		test_of = (off_t)i * BS + SKEW;
> 		fprintf(stderr, "testing %llx...\r", test_of);
> 		if ((r = pthread_rwlock_unlock(&rw_lock_1))) pthread_error(r);
> 		sched_yield();
> 		if (pwrite(h_orig, np, BS, test_of) != BS) {
> 			fprintf(stderr, "can't write (%d): %s\n", r, strerror(errno));
> 			exit(1);
> 		}
> 		if ((r = pthread_rwlock_wrlock(&rw_lock_2))) pthread_error(r);
> 		if ((r = pthread_rwlock_unlock(&rw_lock_3))) pthread_error(r);
> 		if ((r = pthread_rwlock_wrlock(&rw_lock_1))) pthread_error(r);
> 		if ((r = pthread_rwlock_unlock(&rw_lock_2))) pthread_error(r);
> 		if ((r = pthread_rwlock_wrlock(&rw_lock_3))) pthread_error(r);
> 	}
> 	fprintf(stderr,"\n");
> 
> 	test_of = -1;
> 	if ((r = pthread_rwlock_unlock(&rw_lock_1))) pthread_error(r);
> 
> 	for (j = 0; j < THREADS; j++) {
> 		if ((r = pthread_join(thr[j], NULL))) pthread_error(r);
> 	}
> 
> 	fprintf(stderr, "TEST PASSED OK.\n");
> 
> 	return 0;
> }
> 
> 

Here I post a patch that fixes the snapshot deadlock. On schedule(), it 
redirects bios on current->bio_list to helper workqueues.

Mikulas


>From f126e182a053ef2e44a3e70b86df84d2b003530b Mon Sep 17 00:00:00 2001
From: Mikulas Patocka <mpatocka@redhat.com>
Date: Tue, 27 May 2014 11:03:36 -0400
Subject: block: flush queued bios when process blocks to avoid deadlock

The block layer uses per-process bio list to avoid recursion in
generic_make_request.  When generic_make_request is called recursively,
the bio is added to current->bio_list and generic_make_request returns
immediately.  The top-level instance of generic_make_request takes bios
from current->bio_list and processes them.

Commit df2cb6daa4 ("block: Avoid deadlocks with bio allocation by
stacking drivers") created a workqueue for every bio set and code
in bio_alloc_bioset() that tries to resolve some low-memory deadlocks by
redirecting bios queued on current->bio_list to the workqueue if the
system is low on memory.  However another deadlock (see below **) may
happen, without any low memory condition, because generic_make_request
is queuing bios to current->bio_list (rather than submitting them).

Fix this deadlock by redirecting any bios on current->bio_list to the
bio_set's rescue workqueue on every schedule call.  Consequently, when
the process blocks on a mutex, the bios queued on current->bio_list are
dispatched to independent workqueus and they can complete without
waiting for the mutex to be available.

Also, now we can remove punt_bios_to_rescuer() and bio_alloc_bioset()'s
calls to it because bio_alloc_bioset() will implicitly punt all bios on
current->bio_list if it performs a blocking allocation.

** Here is the dm-snapshot deadlock that was observed:

1) Process A sends one-page read bio to the dm-snapshot target. The bio
spans snapshot chunk boundary and so it is split to two bios by device
mapper.

2) Device mapper creates the first sub-bio and sends it to the snapshot
driver.

3) The function snapshot_map calls track_chunk (that allocates a structure
dm_snap_tracked_chunk and adds it to tracked_chunk_hash) and then remaps
the bio to the underlying device and exits with DM_MAPIO_REMAPPED.

4) The remapped bio is submitted with generic_make_request, but it isn't
issued - it is added to current->bio_list instead.

5) Meanwhile, process B (dm's kcopyd) executes pending_complete for the
chunk affected be the first remapped bio, it takes down_write(&s->lock)
and then loops in __check_for_conflicting_io, waiting for
dm_snap_tracked_chunk created in step 3) to be released.

6) Process A continues, it creates a second sub-bio for the rest of the
original bio.

7) snapshot_map is called for this new bio, it waits on
down_write(&s->lock) that is held by Process B (in step 5).

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1267650
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Depends-on: df2cb6daa4 ("block: Avoid deadlocks with bio allocation by stacking drivers")
Cc: stable@vger.kernel.org

---
 block/bio.c            |   77 +++++++++++++++++++------------------------------
 include/linux/blkdev.h |   24 ++++++++++-----
 kernel/sched/core.c    |    7 +---
 3 files changed, 50 insertions(+), 58 deletions(-)


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Mike Snitzer - Jan. 6, 2017, 7:52 p.m.
On Fri, Jan 06 2017 at 12:34pm -0500,
Mikulas Patocka <mpatocka@redhat.com> wrote:

> 
> 
> On Fri, 6 Jan 2017, Mikulas Patocka wrote:
> 
> > 
> > 
> > On Wed, 4 Jan 2017, Mike Snitzer wrote:
> > 
> > > On Wed, Jan 04 2017 at 12:12am -0500,
> > > NeilBrown <neilb@suse.com> wrote:
> > > 
> > > > > Suggested-by: NeilBrown <neilb@suse.com>
> > > > > Signed-off-by: Jack Wang <jinpu.wang@profitbricks.com>
> > > > > ---
> > > > >  block/blk-core.c | 20 ++++++++++++++++++++
> > > > >  1 file changed, 20 insertions(+)
> > > > >
> > > > > diff --git a/block/blk-core.c b/block/blk-core.c
> > > > > index 9e3ac56..47ef373 100644
> > > > > --- a/block/blk-core.c
> > > > > +++ b/block/blk-core.c
> > > > > @@ -2138,10 +2138,30 @@ blk_qc_t generic_make_request(struct bio *bio)
> > > > >  		struct request_queue *q = bdev_get_queue(bio->bi_bdev);
> > > > >  
> > > > >  		if (likely(blk_queue_enter(q, __GFP_DIRECT_RECLAIM) == 0)) {
> > > > > +			struct bio_list lower, same, hold;
> > > > > +
> > > > > +			/* Create a fresh bio_list for all subordinate requests */
> > > > > +			bio_list_init(&hold);
> > > > > +			bio_list_merge(&hold, &bio_list_on_stack);
> > > > > +			bio_list_init(&bio_list_on_stack);
> > > > >  
> > > > >  			ret = q->make_request_fn(q, bio);
> > > > >  
> > > > >  			blk_queue_exit(q);
> > > > > +			/* sort new bios into those for a lower level
> > > > > +			 * and those for the same level
> > > > > +			 */
> > > > > +			bio_list_init(&lower);
> > > > > +			bio_list_init(&same);
> > > > > +			while ((bio = bio_list_pop(&bio_list_on_stack)) != NULL)
> > > > > +				if (q == bdev_get_queue(bio->bi_bdev))
> > > > > +					bio_list_add(&same, bio);
> > > > > +				else
> > > > > +					bio_list_add(&lower, bio);
> > > > > +			/* now assemble so we handle the lowest level first */
> > > > > +			bio_list_merge(&bio_list_on_stack, &lower);
> > > > > +			bio_list_merge(&bio_list_on_stack, &same);
> > > > > +			bio_list_merge(&bio_list_on_stack, &hold);
> > > > >  
> > > > >  			bio = bio_list_pop(current->bio_list);
> > > > >  		} else {
> > > > > -- 
> > > > > 2.7.4
> > > 
> > > Mikulas, would you be willing to try the below patch with the
> > > dm-snapshot deadlock scenario and report back on whether it fixes that?
> > > 
> > > Patch below looks to be the same as here:
> > > https://marc.info/?l=linux-raid&m=148232453107685&q=p3
> > > 
> > > Neil and/or others if that isn't the patch that should be tested please
> > > provide a pointer to the latest.
> > > 
> > > Thanks,
> > > Mike
> > 
> > The bad news is that this doesn't fix the snapshot deadlock.
> > 
> > I created a test program for the snapshot deadlock bug (it was originally 
> > created years ago to test for a different bug, so it contains some cruft). 
> > You also need to insert "if (ci->sector_count) msleep(100);" to the end of 
> > __split_and_process_non_flush to make the kernel sleep when splitting the 
> > bio.
> > 
> > And with the above above patch, the snapshot deadlock bug still happens.

That is really unfortunate.  Would be useful to dig in and understand
why.  Because ordering of the IO in generic_make_request() really should
take care of it.

<snip>
 
> Here I post a patch that fixes the snapshot deadlock. On schedule(), it 
> redirects bios on current->bio_list to helper workqueues.

<snip old patch>

That patch is included in the series of changes sequenced at the top of
this git branch:
http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/log/?h=wip

At the risk of repeating myself: unfortunately it doesn't have a way
forward with the timed offload implementation (which was done to appease
Ming Lei's concern about context switching causing reduced plugging that
results in less efficient IO).

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Patch

Index: linux-4.9-rc3/block/bio.c
===================================================================
--- linux-4.9-rc3.orig/block/bio.c	2016-11-02 23:05:03.000000000 +0100
+++ linux-4.9-rc3/block/bio.c	2016-11-02 23:05:21.000000000 +0100
@@ -353,35 +353,37 @@  static void bio_alloc_rescue(struct work
 	}
 }
 
-static void punt_bios_to_rescuer(struct bio_set *bs)
+/**
+ * blk_flush_bio_list
+ * @tsk: task_struct whose bio_list must be flushed
+ *
+ * Pop bios queued on @tsk->bio_list and submit each of them to
+ * their rescue workqueue.
+ *
+ * If the bio doesn't have a bio_set, we leave it on @tsk->bio_list.
+ * If the bio is allocated from fs_bio_set, we must leave it to avoid
+ * deadlock on loopback block device.
+ * Stacking bio drivers should use bio_set, so this shouldn't be
+ * an issue.
+ */
+void blk_flush_bio_list(struct task_struct *tsk)
 {
-	struct bio_list punt, nopunt;
 	struct bio *bio;
+	struct bio_list list = *tsk->bio_list;
+	bio_list_init(tsk->bio_list);
 
-	/*
-	 * In order to guarantee forward progress we must punt only bios that
-	 * were allocated from this bio_set; otherwise, if there was a bio on
-	 * there for a stacking driver higher up in the stack, processing it
-	 * could require allocating bios from this bio_set, and doing that from
-	 * our own rescuer would be bad.
-	 *
-	 * Since bio lists are singly linked, pop them all instead of trying to
-	 * remove from the middle of the list:
-	 */
-
-	bio_list_init(&punt);
-	bio_list_init(&nopunt);
-
-	while ((bio = bio_list_pop(current->bio_list)))
-		bio_list_add(bio->bi_pool == bs ? &punt : &nopunt, bio);
-
-	*current->bio_list = nopunt;
-
-	spin_lock(&bs->rescue_lock);
-	bio_list_merge(&bs->rescue_list, &punt);
-	spin_unlock(&bs->rescue_lock);
+	while ((bio = bio_list_pop(&list))) {
+		struct bio_set *bs = bio->bi_pool;
+		if (unlikely(!bs) || bs == fs_bio_set) {
+			bio_list_add(tsk->bio_list, bio);
+			continue;
+		}
 
-	queue_work(bs->rescue_workqueue, &bs->rescue_work);
+		spin_lock(&bs->rescue_lock);
+		bio_list_add(&bs->rescue_list, bio);
+		queue_work(bs->rescue_workqueue, &bs->rescue_work);
+		spin_unlock(&bs->rescue_lock);
+	}
 }
 
 /**
@@ -421,7 +423,6 @@  static void punt_bios_to_rescuer(struct 
  */
 struct bio *bio_alloc_bioset(gfp_t gfp_mask, int nr_iovecs, struct bio_set *bs)
 {
-	gfp_t saved_gfp = gfp_mask;
 	unsigned front_pad;
 	unsigned inline_vecs;
 	struct bio_vec *bvl = NULL;
@@ -455,23 +456,11 @@  struct bio *bio_alloc_bioset(gfp_t gfp_m
 		 * reserve.
 		 *
 		 * We solve this, and guarantee forward progress, with a rescuer
-		 * workqueue per bio_set. If we go to allocate and there are
-		 * bios on current->bio_list, we first try the allocation
-		 * without __GFP_DIRECT_RECLAIM; if that fails, we punt those
-		 * bios we would be blocking to the rescuer workqueue before
-		 * we retry with the original gfp_flags.
+		 * workqueue per bio_set. If an allocation would block (due to
+		 * __GFP_DIRECT_RECLAIM) the scheduler will first punt all bios
+		 * on current->bio_list to the rescuer workqueue.
 		 */
-
-		if (current->bio_list && !bio_list_empty(current->bio_list))
-			gfp_mask &= ~__GFP_DIRECT_RECLAIM;
-
 		p = mempool_alloc(bs->bio_pool, gfp_mask);
-		if (!p && gfp_mask != saved_gfp) {
-			punt_bios_to_rescuer(bs);
-			gfp_mask = saved_gfp;
-			p = mempool_alloc(bs->bio_pool, gfp_mask);
-		}
-
 		front_pad = bs->front_pad;
 		inline_vecs = BIO_INLINE_VECS;
 	}
@@ -486,12 +475,6 @@  struct bio *bio_alloc_bioset(gfp_t gfp_m
 		unsigned long idx = 0;
 
 		bvl = bvec_alloc(gfp_mask, nr_iovecs, &idx, bs->bvec_pool);
-		if (!bvl && gfp_mask != saved_gfp) {
-			punt_bios_to_rescuer(bs);
-			gfp_mask = saved_gfp;
-			bvl = bvec_alloc(gfp_mask, nr_iovecs, &idx, bs->bvec_pool);
-		}
-
 		if (unlikely(!bvl))
 			goto err_free;
 
Index: linux-4.9-rc3/include/linux/blkdev.h
===================================================================
--- linux-4.9-rc3.orig/include/linux/blkdev.h	2016-11-02 23:05:03.000000000 +0100
+++ linux-4.9-rc3/include/linux/blkdev.h	2016-11-02 23:05:21.000000000 +0100
@@ -1118,6 +1118,22 @@  static inline bool blk_needs_flush_plug(
 		 !list_empty(&plug->cb_list));
 }
 
+extern void blk_flush_bio_list(struct task_struct *tsk);
+
+static inline void blk_flush_queued_io(struct task_struct *tsk)
+{
+	/*
+	 * Flush any queued bios to corresponding rescue threads.
+	 */
+	if (tsk->bio_list && !bio_list_empty(tsk->bio_list))
+		blk_flush_bio_list(tsk);
+	/*
+	 * Flush any plugged IO that is queued.
+	 */
+	if (blk_needs_flush_plug(tsk))
+		blk_schedule_flush_plug(tsk);
+}
+
 /*
  * tag stuff
  */
@@ -1729,16 +1745,10 @@  static inline void blk_flush_plug(struct
 {
 }
 
-static inline void blk_schedule_flush_plug(struct task_struct *task)
+static inline void blk_flush_queued_io(struct task_struct *tsk)
 {
 }
 
-
-static inline bool blk_needs_flush_plug(struct task_struct *tsk)
-{
-	return false;
-}
-
 static inline int blkdev_issue_flush(struct block_device *bdev, gfp_t gfp_mask,
 				     sector_t *error_sector)
 {
Index: linux-4.9-rc3/kernel/sched/core.c
===================================================================
--- linux-4.9-rc3.orig/kernel/sched/core.c	2016-11-02 23:05:03.000000000 +0100
+++ linux-4.9-rc3/kernel/sched/core.c	2016-11-02 23:05:21.000000000 +0100
@@ -3440,11 +3440,10 @@  static inline void sched_submit_work(str
 	if (!tsk->state || tsk_is_pi_blocked(tsk))
 		return;
 	/*
-	 * If we are going to sleep and we have plugged IO queued,
+	 * If we are going to sleep and we have queued IO,
 	 * make sure to submit it to avoid deadlocks.
 	 */
-	if (blk_needs_flush_plug(tsk))
-		blk_schedule_flush_plug(tsk);
+	blk_flush_queued_io(tsk);
 }
 
 asmlinkage __visible void __sched schedule(void)
@@ -5067,7 +5066,7 @@  long __sched io_schedule_timeout(long ti
 	long ret;
 
 	current->in_iowait = 1;
-	blk_schedule_flush_plug(current);
+	blk_flush_queued_io(current);
 
 	delayacct_blkio_start();
 	rq = raw_rq();