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loop: use worker per cgroup instead of kworker

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Patch series "Charge loop device i/o to issuing cgroup", v14.

The loop device runs all i/o to the backing file on a separate kworker
thread which results in all i/o being charged to the root cgroup.  This
allows a loop device to be used to trivially bypass resource limits and
other policy.  This patch series fixes this gap in accounting.

A simple script to demonstrate this behavior on cgroupv2 machine:

'''
#!/bin/bash
set -e

CGROUP=/sys/fs/cgroup/test.slice
LOOP_DEV=/dev/loop0

if [[ ! -d $CGROUP ]]
then
    sudo mkdir $CGROUP
fi

grep oom_kill $CGROUP/memory.events

# Set a memory limit, write more than that limit to tmpfs -> OOM kill
sudo unshare -m bash -c "
echo \$\$ > $CGROUP/cgroup.procs;
echo 0 > $CGROUP/memory.swap.max;
echo 64M > $CGROUP/memory.max;
mount -t tmpfs -o size=512m tmpfs /tmp;
dd if=/dev/zero of=/tmp/file bs=1M count=256" || true

grep oom_kill $CGROUP/memory.events

# Set a memory limit, write more than that limit through loopback
# device -> no OOM kill
sudo unshare -m bash -c "
echo \$\$ > $CGROUP/cgroup.procs;
echo 0 > $CGROUP/memory.swap.max;
echo 64M > $CGROUP/memory.max;
mount -t tmpfs -o size=512m tmpfs /tmp;
truncate -s 512m /tmp/backing_file
losetup $LOOP_DEV /tmp/backing_file
dd if=/dev/zero of=$LOOP_DEV bs=1M count=256;
losetup -D $LOOP_DEV" || true

grep oom_kill $CGROUP/memory.events
'''

Naively charging cgroups could result in priority inversions through the
single kworker thread in the case where multiple cgroups are
reading/writing to the same loop device.  This patch series does some
minor modification to the loop driver so that each cgroup can make forward
progress independently to avoid this inversion.

With this patch series applied, the above script triggers OOM kills when
writing through the loop device as expected.

This patch (of 3):

Existing uses of loop device may have multiple cgroups reading/writing to
the same device.  Simply charging resources for I/O to the backing file
could result in priority inversion where one cgroup gets synchronously
blocked, holding up all other I/O to the loop device.

In order to avoid this priority inversion, we use a single workqueue where
each work item is a "struct loop_worker" which contains a queue of struct
loop_cmds to issue.  The loop device maintains a tree mapping blk css_id
-> loop_worker.  This allows each cgroup to independently make forward
progress issuing I/O to the backing file.

There is also a single queue for I/O associated with the rootcg which can
be used in cases of extreme memory shortage where we cannot allocate a
loop_worker.

The locking for the tree and queues is fairly heavy handed - we acquire a
per-loop-device spinlock any time either is accessed.  The existing
implementation serializes all I/O through a single thread anyways, so I
don't believe this is any worse.

[colin.king@canonical.com: fixes]

Link: https://lkml.kernel.org/r/20210610173944.1203706-1-schatzberg.dan@gmail.com
Link: https://lkml.kernel.org/r/20210610173944.1203706-2-schatzberg.dan@gmail.com
Signed-off-by: Dan Schatzberg <schatzberg.dan@gmail.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Acked-by: Jens Axboe <axboe@kernel.dk>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Chris Down <chris@chrisdown.name>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Dan Schatzberg 2021-06-28 19:38:15 -07:00 committed by Linus Torvalds
parent b51478a0b3
commit 87579e9b7d
2 changed files with 191 additions and 38 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

217
drivers/block/loop.c
View File

@ -71,7 +71,6 @@
#include <linux/writeback.h>
#include <linux/completion.h>
#include <linux/highmem.h>
#include <linux/kthread.h>
#include <linux/splice.h>
#include <linux/sysfs.h>
#include <linux/miscdevice.h>
@ -84,6 +83,8 @@
#include <linux/uaccess.h>
#define LOOP_IDLE_WORKER_TIMEOUT (60 * HZ)
static DEFINE_IDR(loop_index_idr);
static DEFINE_MUTEX(loop_ctl_mutex);
@ -921,27 +922,95 @@ static void loop_config_discard(struct loop_device *lo)
q->limits.discard_alignment = 0;
}
static void loop_unprepare_queue(struct loop_device *lo)
{
kthread_flush_worker(&lo->worker);
kthread_stop(lo->worker_task);
}
struct loop_worker {
struct rb_node rb_node;
struct work_struct work;
struct list_head cmd_list;
struct list_head idle_list;
struct loop_device *lo;
struct cgroup_subsys_state *css;
unsigned long last_ran_at;
};
static int loop_kthread_worker_fn(void *worker_ptr)
{
current->flags |= PF_LOCAL_THROTTLE | PF_MEMALLOC_NOIO;
return kthread_worker_fn(worker_ptr);
}
static void loop_workfn(struct work_struct *work);
static void loop_rootcg_workfn(struct work_struct *work);
static void loop_free_idle_workers(struct timer_list *timer);
static int loop_prepare_queue(struct loop_device *lo)
#ifdef CONFIG_BLK_CGROUP
static inline int queue_on_root_worker(struct cgroup_subsys_state *css)
{
kthread_init_worker(&lo->worker);
lo->worker_task = kthread_run(loop_kthread_worker_fn,
&lo->worker, "loop%d", lo->lo_number);
if (IS_ERR(lo->worker_task))
return -ENOMEM;
set_user_nice(lo->worker_task, MIN_NICE);
return 0;
return !css || css == blkcg_root_css;
}
#else
static inline int queue_on_root_worker(struct cgroup_subsys_state *css)
{
return !css;
}
#endif
static void loop_queue_work(struct loop_device *lo, struct loop_cmd *cmd)
{
struct rb_node **node = &(lo->worker_tree.rb_node), *parent = NULL;
struct loop_worker *cur_worker, *worker = NULL;
struct work_struct *work;
struct list_head *cmd_list;
spin_lock_irq(&lo->lo_work_lock);
if (queue_on_root_worker(cmd->css))
goto queue_work;
node = &lo->worker_tree.rb_node;
while (*node) {
parent = *node;
cur_worker = container_of(*node, struct loop_worker, rb_node);
if (cur_worker->css == cmd->css) {
worker = cur_worker;
break;
} else if ((long)cur_worker->css < (long)cmd->css) {
node = &(*node)->rb_left;
} else {
node = &(*node)->rb_right;
}
}
if (worker)
goto queue_work;
worker = kzalloc(sizeof(struct loop_worker), GFP_NOWAIT | __GFP_NOWARN);
/*
* In the event we cannot allocate a worker, just queue on the
* rootcg worker
*/
if (!worker)
goto queue_work;
worker->css = cmd->css;
css_get(worker->css);
INIT_WORK(&worker->work, loop_workfn);
INIT_LIST_HEAD(&worker->cmd_list);
INIT_LIST_HEAD(&worker->idle_list);
worker->lo = lo;
rb_link_node(&worker->rb_node, parent, node);
rb_insert_color(&worker->rb_node, &lo->worker_tree);
queue_work:
if (worker) {
/*
* We need to remove from the idle list here while
* holding the lock so that the idle timer doesn't
* free the worker
*/
if (!list_empty(&worker->idle_list))
list_del_init(&worker->idle_list);
work = &worker->work;
cmd_list = &worker->cmd_list;
} else {
work = &lo->rootcg_work;
cmd_list = &lo->rootcg_cmd_list;
}
list_add_tail(&cmd->list_entry, cmd_list);
queue_work(lo->workqueue, work);
spin_unlock_irq(&lo->lo_work_lock);
}
static void loop_update_rotational(struct loop_device *lo)
@ -1127,12 +1196,23 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
!file->f_op->write_iter)
lo->lo_flags |= LO_FLAGS_READ_ONLY;
error = loop_prepare_queue(lo);
if (error)
lo->workqueue = alloc_workqueue("loop%d",
WQ_UNBOUND | WQ_FREEZABLE,
0,
lo->lo_number);
if (!lo->workqueue) {
error = -ENOMEM;
goto out_unlock;
}
set_disk_ro(lo->lo_disk, (lo->lo_flags & LO_FLAGS_READ_ONLY) != 0);
INIT_WORK(&lo->rootcg_work, loop_rootcg_workfn);
INIT_LIST_HEAD(&lo->rootcg_cmd_list);
INIT_LIST_HEAD(&lo->idle_worker_list);
lo->worker_tree = RB_ROOT;
timer_setup(&lo->timer, loop_free_idle_workers,
TIMER_DEFERRABLE);
lo->use_dio = lo->lo_flags & LO_FLAGS_DIRECT_IO;
lo->lo_device = bdev;
lo->lo_backing_file = file;
@ -1200,6 +1280,7 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
int err = 0;
bool partscan = false;
int lo_number;
struct loop_worker *pos, *worker;
mutex_lock(&lo->lo_mutex);
if (WARN_ON_ONCE(lo->lo_state != Lo_rundown)) {
@ -1219,6 +1300,18 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
/* freeze request queue during the transition */
blk_mq_freeze_queue(lo->lo_queue);
destroy_workqueue(lo->workqueue);
spin_lock_irq(&lo->lo_work_lock);
list_for_each_entry_safe(worker, pos, &lo->idle_worker_list,
idle_list) {
list_del(&worker->idle_list);
rb_erase(&worker->rb_node, &lo->worker_tree);
css_put(worker->css);
kfree(worker);
}
spin_unlock_irq(&lo->lo_work_lock);
del_timer_sync(&lo->timer);
spin_lock_irq(&lo->lo_lock);
lo->lo_backing_file = NULL;
spin_unlock_irq(&lo->lo_lock);
@ -1255,7 +1348,6 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
partscan = lo->lo_flags & LO_FLAGS_PARTSCAN && bdev;
lo_number = lo->lo_number;
loop_unprepare_queue(lo);
out_unlock:
mutex_unlock(&lo->lo_mutex);
if (partscan) {
@ -2015,7 +2107,7 @@ static blk_status_t loop_queue_rq(struct blk_mq_hw_ctx *hctx,
} else
#endif
cmd->css = NULL;
kthread_queue_work(&lo->worker, &cmd->work);
loop_queue_work(lo, cmd);
return BLK_STS_OK;
}
@ -2045,26 +2137,82 @@ static void loop_handle_cmd(struct loop_cmd *cmd)
}
}
static void loop_queue_work(struct kthread_work *work)
static void loop_set_timer(struct loop_device *lo)
{
struct loop_cmd *cmd =
container_of(work, struct loop_cmd, work);
loop_handle_cmd(cmd);
timer_reduce(&lo->timer, jiffies + LOOP_IDLE_WORKER_TIMEOUT);
}
static int loop_init_request(struct blk_mq_tag_set *set, struct request *rq,
unsigned int hctx_idx, unsigned int numa_node)
static void loop_process_work(struct loop_worker *worker,
struct list_head *cmd_list, struct loop_device *lo)
{
struct loop_cmd *cmd = blk_mq_rq_to_pdu(rq);
int orig_flags = current->flags;
struct loop_cmd *cmd;
kthread_init_work(&cmd->work, loop_queue_work);
return 0;
current->flags |= PF_LOCAL_THROTTLE | PF_MEMALLOC_NOIO;
spin_lock_irq(&lo->lo_work_lock);
while (!list_empty(cmd_list)) {
cmd = container_of(
cmd_list->next, struct loop_cmd, list_entry);
list_del(cmd_list->next);
spin_unlock_irq(&lo->lo_work_lock);
loop_handle_cmd(cmd);
cond_resched();
spin_lock_irq(&lo->lo_work_lock);
}
/*
* We only add to the idle list if there are no pending cmds
* *and* the worker will not run again which ensures that it
* is safe to free any worker on the idle list
*/
if (worker && !work_pending(&worker->work)) {
worker->last_ran_at = jiffies;
list_add_tail(&worker->idle_list, &lo->idle_worker_list);
loop_set_timer(lo);
}
spin_unlock_irq(&lo->lo_work_lock);
current->flags = orig_flags;
}
static void loop_workfn(struct work_struct *work)
{
struct loop_worker *worker =
container_of(work, struct loop_worker, work);
loop_process_work(worker, &worker->cmd_list, worker->lo);
}
static void loop_rootcg_workfn(struct work_struct *work)
{
struct loop_device *lo =
container_of(work, struct loop_device, rootcg_work);
loop_process_work(NULL, &lo->rootcg_cmd_list, lo);
}
static void loop_free_idle_workers(struct timer_list *timer)
{
struct loop_device *lo = container_of(timer, struct loop_device, timer);
struct loop_worker *pos, *worker;
spin_lock_irq(&lo->lo_work_lock);
list_for_each_entry_safe(worker, pos, &lo->idle_worker_list,
idle_list) {
if (time_is_after_jiffies(worker->last_ran_at +
LOOP_IDLE_WORKER_TIMEOUT))
break;
list_del(&worker->idle_list);
rb_erase(&worker->rb_node, &lo->worker_tree);
css_put(worker->css);
kfree(worker);
}
if (!list_empty(&lo->idle_worker_list))
loop_set_timer(lo);
spin_unlock_irq(&lo->lo_work_lock);
}
static const struct blk_mq_ops loop_mq_ops = {
.queue_rq = loop_queue_rq,
.init_request = loop_init_request,
.complete = lo_complete_rq,
};
@ -2153,6 +2301,7 @@ static int loop_add(struct loop_device **l, int i)
mutex_init(&lo->lo_mutex);
lo->lo_number = i;
spin_lock_init(&lo->lo_lock);
spin_lock_init(&lo->lo_work_lock);
disk->major = LOOP_MAJOR;
disk->first_minor = i << part_shift;
disk->fops = &lo_fops;

12
drivers/block/loop.h
View File

@ -14,7 +14,6 @@
#include <linux/blk-mq.h>
#include <linux/spinlock.h>
#include <linux/mutex.h>
#include <linux/kthread.h>
#include <uapi/linux/loop.h>
/* Possible states of device */
@ -55,8 +54,13 @@ struct loop_device {
spinlock_t lo_lock;
int lo_state;
struct kthread_worker worker;
struct task_struct *worker_task;
spinlock_t lo_work_lock;
struct workqueue_struct *workqueue;
struct work_struct rootcg_work;
struct list_head rootcg_cmd_list;
struct list_head idle_worker_list;
struct rb_root worker_tree;
struct timer_list timer;
bool use_dio;
bool sysfs_inited;
@ -67,7 +71,7 @@ struct loop_device {
};
struct loop_cmd {
struct kthread_work work;
struct list_head list_entry;
bool use_aio; /* use AIO interface to handle I/O */
atomic_t ref; /* only for aio */
long ret;