[PATCH 3/3] ubifs: Don't stop retrying even if committing times exceeds two

From: Zhihao Cheng
Date: Sun Nov 19 2023 - 22:20:00 EST

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
(pid 3640152) 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 writeback threads, which fails
finding dirty LEBs of GC in current thread, so make_reservation() should
try many times to invoke GC&&committing, but current realization limits
the times of retrying as 'nospc_retries'(twice). Actually the limitation
of retrying times should be removed from make_reservation(), but
considering the existence of 'cmt_retries' in case of infinite loop,
just remove 'nospc_retries' and set the limitation of retrying times same
as 'cmt_retries'.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=218164
Fixes: 1e51764a3c2a ("UBIFS: add new flash file system")
Signed-off-by: Zhihao Cheng <chengzhihao1@xxxxxxxxxx>
---
fs/ubifs/journal.c | 26 ++++++++++----------------
1 file changed, 10 insertions(+), 16 deletions(-)

diff --git a/fs/ubifs/journal.c b/fs/ubifs/journal.c
index f0a5538c84b0..ec42d42f06dc 100644
--- a/fs/ubifs/journal.c
+++ b/fs/ubifs/journal.c
@@ -310,7 +310,7 @@ 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;
+ int err, cmt_retries = 0;

again:
down_read(&c->commit_sem);
@@ -323,21 +323,14 @@ static int make_reservation(struct ubifs_info *c, int jhead, int len)
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
- * 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.
+ * because it could make some dirty space and GC would make
+ * progress, so that the below will commit and re-try. This
+ * process could repeat many times(cmt_retries < 128),
+ * journal heads in other threads could grab the dirty lebs,
+ * which fails finding dirty LEBs of GC in current thread.
*/
+ dbg_jnl("no space, retry");
+ err = -EAGAIN;
}

if (err != -EAGAIN)
@@ -350,7 +343,8 @@ 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.
+ * are too tough, or the racing between GC and journal heads
+ * switching is too frequent when space is nearly run out.
*/
ubifs_err(c, "stuck in space allocation");
err = -ENOSPC;
--
2.39.2