[PATCH 2/2] erofs: sequence each shrink task

[Guowei Du]

From: duguowei <duguowei at xiaomi.com>

Because of 'list_move_tail', if two or more tasks are shrinking, there
will be different results for them.

For example:
After the first round, if shrink_run_no of entry equals run_no of task,
task will break directly at the beginning of next round; if they are
not equal, task will continue to shrink until encounter one entry
which has the same number.

It is difficult to confirm the real results of all tasks, so add a lock
to only allow one task to shrink at the same time.

How to test:
task1:
root#echo 3 > /proc/sys/vm/drop_caches
[743071.839051] Call Trace:
[743071.839052]  <TASK>
[743071.839054]  do_shrink_slab+0x112/0x300
[743071.839058]  shrink_slab+0x211/0x2a0
[743071.839060]  drop_slab+0x72/0xe0
[743071.839061]  drop_caches_sysctl_handler+0x50/0xb0
[743071.839063]  proc_sys_call_handler+0x173/0x250
[743071.839066]  proc_sys_write+0x13/0x20
[743071.839067]  new_sync_write+0x104/0x180
[743071.839070]  ? send_command+0xe0/0x270
[743071.839073]  vfs_write+0x247/0x2a0
[743071.839074]  ksys_write+0xa7/0xe0
[743071.839075]  ? fpregs_assert_state_consistent+0x23/0x50
[743071.839078]  __x64_sys_write+0x1a/0x20
[743071.839079]  do_syscall_64+0x3a/0x80
[743071.839081]  entry_SYSCALL_64_after_hwframe+0x46/0xb0

task2:
root#echo 3 > /proc/sys/vm/drop_caches
[743079.843214] Call Trace:
[743079.843214]  <TASK>
[743079.843215]  do_shrink_slab+0x112/0x300
[743079.843219]  shrink_slab+0x211/0x2a0
[743079.843221]  drop_slab+0x72/0xe0
[743079.843222]  drop_caches_sysctl_handler+0x50/0xb0
[743079.843224]  proc_sys_call_handler+0x173/0x250
[743079.843227]  proc_sys_write+0x13/0x20
[743079.843228]  new_sync_write+0x104/0x180
[743079.843231]  ? send_command+0xe0/0x270
[743079.843233]  vfs_write+0x247/0x2a0
[743079.843234]  ksys_write+0xa7/0xe0
[743079.843235]  ? fpregs_assert_state_consistent+0x23/0x50
[743079.843238]  __x64_sys_write+0x1a/0x20
[743079.843239]  do_syscall_64+0x3a/0x80
[743079.843241]  entry_SYSCALL_64_after_hwframe+0x46/0xb0

Signed-off-by: duguowei <duguowei at xiaomi.com>
---
 fs/erofs/utils.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/fs/erofs/utils.c b/fs/erofs/utils.c
index ec9a1d780dc1..9eca13a7e594 100644
--- a/fs/erofs/utils.c
+++ b/fs/erofs/utils.c
@@ -186,6 +186,8 @@ static unsigned int shrinker_run_no;
 
 /* protects the mounted 'erofs_sb_list' */
 static DEFINE_SPINLOCK(erofs_sb_list_lock);
+/* sequence each shrink task */
+static DEFINE_SPINLOCK(erofs_sb_shrink_lock);
 static LIST_HEAD(erofs_sb_list);
 
 void erofs_shrinker_register(struct super_block *sb)
@@ -226,13 +228,14 @@ static unsigned long erofs_shrink_scan(struct shrinker *shrink,
 	struct list_head *p;
 
 	unsigned long nr = sc->nr_to_scan;
-	unsigned int run_no;
 	unsigned long freed = 0;
 
+	spin_lock(&erofs_sb_shrink_lock);
 	spin_lock(&erofs_sb_list_lock);
-	do {
-		run_no = ++shrinker_run_no;
-	} while (run_no == 0);
+	shrinker_run_no++;
+	/* if overflow, restarting from 1 */
+	if (shrinker_run_no == 0)
+		shrinker_run_no = 1;
 
 	/* Iterate over all mounted superblocks and try to shrink them */
 	p = erofs_sb_list.next;
@@ -243,7 +246,7 @@ static unsigned long erofs_shrink_scan(struct shrinker *shrink,
 		 * We move the ones we do to the end of the list, so we stop
 		 * when we see one we have already done.
 		 */
-		if (sbi->shrinker_run_no == run_no)
+		if (sbi->shrinker_run_no == shrinker_run_no)
 			break;
 
 		if (!mutex_trylock(&sbi->umount_mutex)) {
@@ -252,7 +255,7 @@ static unsigned long erofs_shrink_scan(struct shrinker *shrink,
 		}
 
 		spin_unlock(&erofs_sb_list_lock);
-		sbi->shrinker_run_no = run_no;
+		sbi->shrinker_run_no = shrinker_run_no;
 
 		freed += erofs_shrink_workstation(sbi, nr - freed);
 
@@ -271,6 +274,7 @@ static unsigned long erofs_shrink_scan(struct shrinker *shrink,
 			break;
 	}
 	spin_unlock(&erofs_sb_list_lock);
+	spin_unlock(&erofs_sb_shrink_lock);
 	return freed;
 }
 
-- 
2.36.1