[PATCH v2] ubifs: Queue up space reservation tasks if retrying many times
Zhihao Cheng
chengzhihao1 at huawei.com
Sun Jan 21 22:31:03 PST 2024
Recently we catched ENOSPC returned by make_reservation() while doing
fsstress on UBIFS, we got following information when it occurred (See
details in Link):
UBIFS error (ubi0:0 pid 3640152): make_reservation [ubifs]: cannot
reserve 112 bytes in jhead 2, error -28
CPU: 2 PID: 3640152 Comm: kworker/u16:2 Tainted: G B W
Hardware name: Hisilicon PhosphorHi1230 EMU (DT)
Workqueue: writeback wb_workfn (flush-ubifs_0_0)
Call trace:
dump_stack+0x114/0x198
make_reservation+0x564/0x610 [ubifs]
ubifs_jnl_write_data+0x328/0x48c [ubifs]
do_writepage+0x2a8/0x3e4 [ubifs]
ubifs_writepage+0x16c/0x374 [ubifs]
generic_writepages+0xb4/0x114
do_writepages+0xcc/0x11c
writeback_sb_inodes+0x2d0/0x564
wb_writeback+0x20c/0x2b4
wb_workfn+0x404/0x510
process_one_work+0x304/0x4ac
worker_thread+0x31c/0x4e4
kthread+0x23c/0x290
Budgeting info: data budget sum 17576, total budget sum 17768
budg_data_growth 4144, budg_dd_growth 13432, budg_idx_growth 192
min_idx_lebs 13, old_idx_sz 988640, uncommitted_idx 0
page_budget 4144, inode_budget 160, dent_budget 312
nospace 0, nospace_rp 0
dark_wm 8192, dead_wm 4096, max_idx_node_sz 192
freeable_cnt 0, calc_idx_sz 988640, idx_gc_cnt 0
dirty_pg_cnt 4, dirty_zn_cnt 0, clean_zn_cnt 4811
gc_lnum 21, ihead_lnum 14
jhead 0 (GC) LEB 16
jhead 1 (base) LEB 34
jhead 2 (data) LEB 23
bud LEB 16
bud LEB 23
bud LEB 34
old bud LEB 33
old bud LEB 31
old bud LEB 15
commit state 4
Budgeting predictions:
available: 33832, outstanding 17576, free 15356
(pid 3640152) start dumping LEB properties
(pid 3640152) Lprops statistics: empty_lebs 3, idx_lebs 11
taken_empty_lebs 1, total_free 1253376, total_dirty 2445736
total_used 3438712, total_dark 65536, total_dead 17248
LEB 15 free 0 dirty 248000 used 5952 (taken)
LEB 16 free 110592 dirty 896 used 142464 (taken, jhead 0 (GC))
LEB 21 free 253952 dirty 0 used 0 (taken, GC LEB)
LEB 23 free 0 dirty 248104 used 5848 (taken, jhead 2 (data))
LEB 29 free 253952 dirty 0 used 0 (empty)
LEB 33 free 0 dirty 253952 used 0 (taken)
LEB 34 free 217088 dirty 36544 used 320 (taken, jhead 1 (base))
LEB 37 free 253952 dirty 0 used 0 (empty)
OTHERS: index lebs, zero-available non-index lebs
According to the budget algorithm, there are 5 LEBs reserved for budget:
three journal heads(16,23,34), 1 GC LEB(21) and 1 deletion LEB(can be
used in make_reservation()). There are 2 empty LEBs used for index nodes,
which is calculated as min_idx_lebs - idx_lebs = 2. In theory, LEB 15
and 33 should be reclaimed as free state after committing, but it is now
in taken state. After looking the realization of reserve_space(), there's
a possible situation:
LEB 15: free 2000 dirty 248000 used 3952 (jhead 2)
LEB 23: free 2000 dirty 248104 used 3848 (bud, taken)
LEB 33: free 2000 dirty 251952 used 0 (bud, taken)
wb_workfn wb_workfn_2
do_writepage // write 3000 bytes
ubifs_jnl_write_data
make_reservation
reserve_space
ubifs_garbage_collect
ubifs_find_dirty_leb // ret ENOSPC, dirty LEBs are taken
nospc_retries++ // 1
ubifs_run_commit
do_commit
LEB 15: free 2000 dirty 248000 used 3952 (jhead 2)
LEB 23: free 2000 dirty 248104 used 3848 (dirty)
LEB 33: free 2000 dirty 251952 used 0 (dirty)
do_writepage // write 2000 bytes for 3 times
ubifs_jnl_write_data
// grabs 15\23\33
LEB 15: free 0 dirty 248000 used 5952 (bud, taken)
LEB 23: free 0 dirty 248104 used 5848 (jhead 2)
LEB 33: free 0 dirty 253952 used 0 (bud, taken)
reserve_space
ubifs_garbage_collect
ubifs_find_dirty_leb // ret ENOSPC, dirty LEBs are taken
if (nospc_retries++ < 2) // false
ubifs_ro_mode !
Fetch a reproducer in Link.
The dirty LEBs could be grabbed by other threads, which fails finding dirty
LEBs of GC in current thread, so make_reservation() could try many times to
invoke GC&&committing, but current realization limits the times of retrying
as 'nospc_retries'(twice).
Fix it by adding a wait queue, start queuing up space reservation tasks
when someone task has retried gc + commit for many times. Then there is
only one task making space reservation at any time, and it can always make
success under the premise of correct budgeting.
Link: https://bugzilla.kernel.org/show_bug.cgi?id=218164
Fixes: 1e51764a3c2a ("UBIFS: add new flash file system")
Signed-off-by: Zhihao Cheng <chengzhihao1 at huawei.com>
Signed-off-by: Zhang Yi <yi.zhang at huawei.com>
---
v1->v2: Change fix method, increasing retrying times => queuing up all space
reservation tasks if someone task has retried many times.
fs/ubifs/journal.c | 171 +++++++++++++++++++++++++++++++++++++++------
fs/ubifs/super.c | 2 +
fs/ubifs/ubifs.h | 4 ++
3 files changed, 155 insertions(+), 22 deletions(-)
diff --git a/fs/ubifs/journal.c b/fs/ubifs/journal.c
index f0a5538c84b0..74aee92433d7 100644
--- a/fs/ubifs/journal.c
+++ b/fs/ubifs/journal.c
@@ -292,6 +292,96 @@ static int write_head(struct ubifs_info *c, int jhead, void *buf, int len,
return err;
}
+/**
+ * __queue_and_wait - queue a task and wait until the task is waked up.
+ * @c: UBIFS file-system description object
+ *
+ * This function adds current task in queue and waits until the task is waked
+ * up. This function should be called with @c->reserve_space_wq locked.
+ */
+static void __queue_and_wait(struct ubifs_info *c)
+{
+ DEFINE_WAIT(wait);
+
+ __add_wait_queue_entry_tail_exclusive(&c->reserve_space_wq, &wait);
+ set_current_state(TASK_UNINTERRUPTIBLE);
+ spin_unlock(&c->reserve_space_wq.lock);
+
+ schedule();
+ finish_wait(&c->reserve_space_wq, &wait);
+}
+
+/**
+ * wait_for_reservation - try queuing current task to wait until waked up.
+ * @c: UBIFS file-system description object
+ *
+ * This function queues current task to wait until waked up, if queuing is
+ * started(@c->need_wait_space is not %0). Returns %true if current task is
+ * added in queue, otherwise %false is returned.
+ */
+static bool wait_for_reservation(struct ubifs_info *c)
+{
+ if (likely(atomic_read(&c->need_wait_space) == 0))
+ /* Quick path to check whether queuing is started. */
+ return false;
+
+ spin_lock(&c->reserve_space_wq.lock);
+ if (atomic_read(&c->need_wait_space) == 0) {
+ /* Queuing is not started, don't queue current task. */
+ spin_unlock(&c->reserve_space_wq.lock);
+ return false;
+ }
+
+ __queue_and_wait(c);
+ return true;
+}
+
+/**
+ * wake_up_reservation - wake up first task in queue or stop queuing.
+ * @c: UBIFS file-system description object
+ *
+ * This function wakes up the first task in queue if it exists, or stops
+ * queuing if no tasks in queue.
+ */
+static void wake_up_reservation(struct ubifs_info *c)
+{
+ spin_lock(&c->reserve_space_wq.lock);
+ if (waitqueue_active(&c->reserve_space_wq))
+ wake_up_locked(&c->reserve_space_wq);
+ else
+ /*
+ * Compared with wait_for_reservation(), set @c->need_wait_space
+ * under the protection of wait queue lock, which can avoid that
+ * @c->need_wait_space is set to 0 after new task queued.
+ */
+ atomic_set(&c->need_wait_space, 0);
+ spin_unlock(&c->reserve_space_wq.lock);
+}
+
+/**
+ * wake_up_reservation - add current task in queue or start queuing.
+ * @c: UBIFS file-system description object
+ *
+ * This function starts queuing if queuing is not started, otherwise adds
+ * current task in queue.
+ */
+static void add_or_start_queue(struct ubifs_info *c)
+{
+ spin_lock(&c->reserve_space_wq.lock);
+ if (atomic_cmpxchg(&c->need_wait_space, 0, 1) == 0) {
+ /* Starts queuing, task can go on directly. */
+ spin_unlock(&c->reserve_space_wq.lock);
+ return;
+ }
+
+ /*
+ * There are at least two tasks have retried more than 32 times
+ * at certain point, first task has started queuing, just queue
+ * the left tasks.
+ */
+ __queue_and_wait(c);
+}
+
/**
* make_reservation - reserve journal space.
* @c: UBIFS file-system description object
@@ -311,33 +401,27 @@ static int write_head(struct ubifs_info *c, int jhead, void *buf, int len,
static int make_reservation(struct ubifs_info *c, int jhead, int len)
{
int err, cmt_retries = 0, nospc_retries = 0;
+ bool blocked = wait_for_reservation(c);
again:
down_read(&c->commit_sem);
err = reserve_space(c, jhead, len);
- if (!err)
+ if (!err) {
/* c->commit_sem will get released via finish_reservation(). */
- return 0;
+ goto out_wake_up;
+ }
up_read(&c->commit_sem);
if (err == -ENOSPC) {
/*
* GC could not make any progress. We should try to commit
- * once because it could make some dirty space and GC would
- * make progress, so make the error -EAGAIN so that the below
+ * because it could make some dirty space and GC would make
+ * progress, so make the error -EAGAIN so that the below
* will commit and re-try.
*/
- if (nospc_retries++ < 2) {
- dbg_jnl("no space, retry");
- err = -EAGAIN;
- }
-
- /*
- * This means that the budgeting is incorrect. We always have
- * to be able to write to the media, because all operations are
- * budgeted. Deletions are not budgeted, though, but we reserve
- * an extra LEB for them.
- */
+ nospc_retries++;
+ dbg_jnl("no space, retry");
+ err = -EAGAIN;
}
if (err != -EAGAIN)
@@ -349,15 +433,37 @@ static int make_reservation(struct ubifs_info *c, int jhead, int len)
*/
if (cmt_retries > 128) {
/*
- * This should not happen unless the journal size limitations
- * are too tough.
+ * This should not happen unless:
+ * 1. The journal size limitations are too tough.
+ * 2. The budgeting is incorrect. We always have to be able to
+ * write to the media, because all operations are budgeted.
+ * Deletions are not budgeted, though, but we reserve an
+ * extra LEB for them.
*/
- ubifs_err(c, "stuck in space allocation");
+ ubifs_err(c, "stuck in space allocation, nospc_retries %d",
+ nospc_retries);
err = -ENOSPC;
goto out;
- } else if (cmt_retries > 32)
- ubifs_warn(c, "too many space allocation re-tries (%d)",
- cmt_retries);
+ } else if (cmt_retries > 32) {
+ /*
+ * It's almost impossible to happen, unless there are many tasks
+ * making reservation concurrently and someone task has retried
+ * gc + commit for many times, generated available space during
+ * this period are grabbed by other tasks.
+ * But if it happens, start queuing up all tasks that will make
+ * space reservation, then there is only one task making space
+ * reservation at any time, and it can always make success under
+ * the premise of correct budgeting.
+ */
+ ubifs_warn(c, "too many space allocation cmt_retries (%d) "
+ "nospc_retries (%d), start queuing tasks",
+ cmt_retries, nospc_retries);
+
+ if (!blocked) {
+ blocked = true;
+ add_or_start_queue(c);
+ }
+ }
dbg_jnl("-EAGAIN, commit and retry (retried %d times)",
cmt_retries);
@@ -365,7 +471,7 @@ static int make_reservation(struct ubifs_info *c, int jhead, int len)
err = ubifs_run_commit(c);
if (err)
- return err;
+ goto out_wake_up;
goto again;
out:
@@ -380,6 +486,27 @@ static int make_reservation(struct ubifs_info *c, int jhead, int len)
cmt_retries = dbg_check_lprops(c);
up_write(&c->commit_sem);
}
+out_wake_up:
+ if (blocked) {
+ /*
+ * Only tasks that have ever started queuing or ever been queued
+ * can wake up other queued tasks, which can make sure that
+ * there is only one task waked up to make space reservation.
+ * For example:
+ * task A task B task C
+ * make_reservation make_reservation
+ * reserve_space // 0
+ * wake_up_reservation
+ * atomic_cmpxchg // 0, start queuing
+ * reserve_space
+ * wait_for_reservation
+ * __queue_and_wait
+ * add_wait_queue
+ * if (blocked) // false
+ * // So that task C won't be waked up to race with task B
+ */
+ wake_up_reservation(c);
+ }
return err;
}
diff --git a/fs/ubifs/super.c b/fs/ubifs/super.c
index 09e270d6ed02..571c9dc92b94 100644
--- a/fs/ubifs/super.c
+++ b/fs/ubifs/super.c
@@ -2151,6 +2151,8 @@ static struct ubifs_info *alloc_ubifs_info(struct ubi_volume_desc *ubi)
mutex_init(&c->bu_mutex);
mutex_init(&c->write_reserve_mutex);
init_waitqueue_head(&c->cmt_wq);
+ init_waitqueue_head(&c->reserve_space_wq);
+ atomic_set(&c->need_wait_space, 0);
c->buds = RB_ROOT;
c->old_idx = RB_ROOT;
c->size_tree = RB_ROOT;
diff --git a/fs/ubifs/ubifs.h b/fs/ubifs/ubifs.h
index ee910c8ce157..7a7c62104f2b 100644
--- a/fs/ubifs/ubifs.h
+++ b/fs/ubifs/ubifs.h
@@ -1047,6 +1047,8 @@ struct ubifs_debug_info;
* @bg_bud_bytes: number of bud bytes when background commit is initiated
* @old_buds: buds to be released after commit ends
* @max_bud_cnt: maximum number of buds
+ * @need_wait_space: Non %0 means space reservation tasks need to wait in queue
+ * @reserve_space_wq: wait queue to sleep on if @need_wait_space is not %0
*
* @commit_sem: synchronizes committer with other processes
* @cmt_state: commit state
@@ -1305,6 +1307,8 @@ struct ubifs_info {
long long bg_bud_bytes;
struct list_head old_buds;
int max_bud_cnt;
+ atomic_t need_wait_space;
+ wait_queue_head_t reserve_space_wq;
struct rw_semaphore commit_sem;
int cmt_state;
--
2.39.2
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