[PATCH -V2 1/3] migrate_pages: fix deadlock in batched migration

From: Huang Ying
Date: Thu Mar 02 2023 - 22:02:37 EST

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Two deadlock bugs were reported for the migrate_pages() batching
series. Thanks Hugh and Pengfei! For example, in the following
deadlock trace snippet,

INFO: task kworker/u4:0:9 blocked for more than 147 seconds.
Not tainted 6.2.0-rc4-kvm+ #1314
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:kworker/u4:0 state:D stack:0 pid:9 ppid:2 flags:0x00004000
Workqueue: loop4 loop_rootcg_workfn
Call Trace:
<TASK>
__schedule+0x43b/0xd00
schedule+0x6a/0xf0
io_schedule+0x4a/0x80
folio_wait_bit_common+0x1b5/0x4e0
? __pfx_wake_page_function+0x10/0x10
__filemap_get_folio+0x73d/0x770
shmem_get_folio_gfp+0x1fd/0xc80
shmem_write_begin+0x91/0x220
generic_perform_write+0x10e/0x2e0
__generic_file_write_iter+0x17e/0x290
? generic_write_checks+0x12b/0x1a0
generic_file_write_iter+0x97/0x180
? __sanitizer_cov_trace_const_cmp4+0x1a/0x20
do_iter_readv_writev+0x13c/0x210
? __sanitizer_cov_trace_const_cmp4+0x1a/0x20
do_iter_write+0xf6/0x330
vfs_iter_write+0x46/0x70
loop_process_work+0x723/0xfe0
loop_rootcg_workfn+0x28/0x40
process_one_work+0x3cc/0x8d0
worker_thread+0x66/0x630
? __pfx_worker_thread+0x10/0x10
kthread+0x153/0x190
? __pfx_kthread+0x10/0x10
ret_from_fork+0x29/0x50
</TASK>

INFO: task repro:1023 blocked for more than 147 seconds.
Not tainted 6.2.0-rc4-kvm+ #1314
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:repro state:D stack:0 pid:1023 ppid:360 flags:0x00004004
Call Trace:
<TASK>
__schedule+0x43b/0xd00
schedule+0x6a/0xf0
io_schedule+0x4a/0x80
folio_wait_bit_common+0x1b5/0x4e0
? compaction_alloc+0x77/0x1150
? __pfx_wake_page_function+0x10/0x10
folio_wait_bit+0x30/0x40
folio_wait_writeback+0x2e/0x1e0
migrate_pages_batch+0x555/0x1ac0
? __pfx_compaction_alloc+0x10/0x10
? __pfx_compaction_free+0x10/0x10
? __this_cpu_preempt_check+0x17/0x20
? lock_is_held_type+0xe6/0x140
migrate_pages+0x100e/0x1180
? __pfx_compaction_free+0x10/0x10
? __pfx_compaction_alloc+0x10/0x10
compact_zone+0xe10/0x1b50
? lock_is_held_type+0xe6/0x140
? check_preemption_disabled+0x80/0xf0
compact_node+0xa3/0x100
? __sanitizer_cov_trace_const_cmp8+0x1c/0x30
? _find_first_bit+0x7b/0x90
sysctl_compaction_handler+0x5d/0xb0
proc_sys_call_handler+0x29d/0x420
proc_sys_write+0x2b/0x40
vfs_write+0x3a3/0x780
ksys_write+0xb7/0x180
__x64_sys_write+0x26/0x30
do_syscall_64+0x3b/0x90
entry_SYSCALL_64_after_hwframe+0x72/0xdc
RIP: 0033:0x7f3a2471f59d
RSP: 002b:00007ffe567f7288 EFLAGS: 00000217 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f3a2471f59d
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000005
RBP: 00007ffe567f72a0 R08: 0000000000000010 R09: 0000000000000010
R10: 0000000000000010 R11: 0000000000000217 R12: 00000000004012e0
R13: 00007ffe567f73e0 R14: 0000000000000000 R15: 0000000000000000
</TASK>

The page migration task has held the lock of the shmem folio A, and is
waiting the writeback of the folio B of the file system on the loop
block device to complete. While the loop worker task which writes
back the folio B is waiting to lock the shmem folio A, because the
folio A backs the folio B in the loop device. Thus deadlock is
triggered.

In general, if we have locked some other folios except the one we are
migrating, it's not safe to wait synchronously, for example, to wait
the writeback to complete or wait to lock the buffer head.

To fix the deadlock, in this patch, we avoid to batch the page
migration except for MIGRATE_ASYNC mode. In MIGRATE_ASYNC mode,
synchronous waiting is avoided.

The fix can be improved further. We will do that as soon as possible.

Link: https://lore.kernel.org/linux-mm/87a6c8c-c5c1-67dc-1e32-eb30831d6e3d@xxxxxxxxxx/
Link: https://lore.kernel.org/linux-mm/874jrg7kke.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
Link: https://lore.kernel.org/linux-mm/20230227110614.dngdub2j3exr6dfp@quack3/
Fixes: 5dfab109d519 ("migrate_pages: batch _unmap and _move")
Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Reported-by: Hugh Dickins <hughd@xxxxxxxxxx>
Reported-by: "Xu, Pengfei" <pengfei.xu@xxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Stefan Roesch <shr@xxxxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Xin Hao <xhao@xxxxxxxxxxxxxxxxx>
Cc: Zi Yan <ziy@xxxxxxxxxx>
Cc: Yang Shi <shy828301@xxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
mm/migrate.c | 69 ++++++++++++++++++++--------------------------------
1 file changed, 26 insertions(+), 43 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 04b9b3eeecef..7d751d6a2c15 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1115,7 +1115,7 @@ static void migrate_folio_done(struct folio *src,
/* Obtain the lock on page, remove all ptes. */
static int migrate_folio_unmap(new_page_t get_new_page, free_page_t put_new_page,
unsigned long private, struct folio *src,
- struct folio **dstp, int force, bool avoid_force_lock,
+ struct folio **dstp, int force,
enum migrate_mode mode, enum migrate_reason reason,
struct list_head *ret)
{
@@ -1166,17 +1166,6 @@ static int migrate_folio_unmap(new_page_t get_new_page, free_page_t put_new_page
if (current->flags & PF_MEMALLOC)
goto out;

- /*
- * We have locked some folios and are going to wait to lock
- * this folio. To avoid a potential deadlock, let's bail
- * out and not do that. The locked folios will be moved and
- * unlocked, then we can wait to lock this folio.
- */
- if (avoid_force_lock) {
- rc = -EDEADLOCK;
- goto out;
- }
-
folio_lock(src);
}
locked = true;
@@ -1256,7 +1245,7 @@ static int migrate_folio_unmap(new_page_t get_new_page, free_page_t put_new_page
/* Establish migration ptes */
VM_BUG_ON_FOLIO(folio_test_anon(src) &&
!folio_test_ksm(src) && !anon_vma, src);
- try_to_migrate(src, TTU_BATCH_FLUSH);
+ try_to_migrate(src, mode == MIGRATE_ASYNC ? TTU_BATCH_FLUSH : 0);
page_was_mapped = 1;
}

@@ -1270,7 +1259,7 @@ static int migrate_folio_unmap(new_page_t get_new_page, free_page_t put_new_page
* A folio that has not been unmapped will be restored to
* right list unless we want to retry.
*/
- if (rc == -EAGAIN || rc == -EDEADLOCK)
+ if (rc == -EAGAIN)
ret = NULL;

migrate_folio_undo_src(src, page_was_mapped, anon_vma, locked, ret);
@@ -1621,6 +1610,11 @@ static int migrate_hugetlbs(struct list_head *from, new_page_t get_new_page,
/*
* migrate_pages_batch() first unmaps folios in the from list as many as
* possible, then move the unmapped folios.
+ *
+ * We only batch migration if mode == MIGRATE_ASYNC to avoid to wait a
+ * lock or bit when we have locked more than one folio. Which may cause
+ * deadlock (e.g., for loop device). So, if mode != MIGRATE_ASYNC, the
+ * length of the from list must be <= 1.
*/
static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
free_page_t put_new_page, unsigned long private,
@@ -1643,11 +1637,11 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
LIST_HEAD(dst_folios);
bool nosplit = (reason == MR_NUMA_MISPLACED);
bool no_split_folio_counting = false;
- bool avoid_force_lock;

+ VM_WARN_ON_ONCE(mode != MIGRATE_ASYNC &&
+ !list_empty(from) && !list_is_singular(from));
retry:
rc_saved = 0;
- avoid_force_lock = false;
retry = 1;
for (pass = 0;
pass < NR_MAX_MIGRATE_PAGES_RETRY && (retry || large_retry);
@@ -1692,15 +1686,14 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
}

rc = migrate_folio_unmap(get_new_page, put_new_page, private,
- folio, &dst, pass > 2, avoid_force_lock,
- mode, reason, ret_folios);
+ folio, &dst, pass > 2, mode,
+ reason, ret_folios);
/*
* The rules are:
* Success: folio will be freed
* Unmap: folio will be put on unmap_folios list,
* dst folio put on dst_folios list
* -EAGAIN: stay on the from list
- * -EDEADLOCK: stay on the from list
* -ENOMEM: stay on the from list
* Other errno: put on ret_folios list
*/
@@ -1752,14 +1745,6 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
goto out;
else
goto move;
- case -EDEADLOCK:
- /*
- * The folio cannot be locked for potential deadlock.
- * Go move (and unlock) all locked folios. Then we can
- * try again.
- */
- rc_saved = rc;
- goto move;
case -EAGAIN:
if (is_large) {
large_retry++;
@@ -1774,11 +1759,6 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
stats->nr_thp_succeeded += is_thp;
break;
case MIGRATEPAGE_UNMAP:
- /*
- * We have locked some folios, don't force lock
- * to avoid deadlock.
- */
- avoid_force_lock = true;
list_move_tail(&folio->lru, &unmap_folios);
list_add_tail(&dst->lru, &dst_folios);
break;
@@ -1903,17 +1883,15 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
*/
list_splice_init(from, ret_folios);
list_splice_init(&split_folios, from);
+ /*
+ * Force async mode to avoid to wait lock or bit when we have
+ * locked more than one folios.
+ */
+ mode = MIGRATE_ASYNC;
no_split_folio_counting = true;
goto retry;
}

- /*
- * We have unlocked all locked folios, so we can force lock now, let's
- * try again.
- */
- if (rc == -EDEADLOCK)
- goto retry;
-
return rc;
}

@@ -1948,7 +1926,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
enum migrate_mode mode, int reason, unsigned int *ret_succeeded)
{
int rc, rc_gather;
- int nr_pages;
+ int nr_pages, batch;
struct folio *folio, *folio2;
LIST_HEAD(folios);
LIST_HEAD(ret_folios);
@@ -1962,6 +1940,11 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
mode, reason, &stats, &ret_folios);
if (rc_gather < 0)
goto out;
+
+ if (mode == MIGRATE_ASYNC)
+ batch = NR_MAX_BATCHED_MIGRATION;
+ else
+ batch = 1;
again:
nr_pages = 0;
list_for_each_entry_safe(folio, folio2, from, lru) {
@@ -1972,11 +1955,11 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
}

nr_pages += folio_nr_pages(folio);
- if (nr_pages > NR_MAX_BATCHED_MIGRATION)
+ if (nr_pages >= batch)
break;
}
- if (nr_pages > NR_MAX_BATCHED_MIGRATION)
- list_cut_before(&folios, from, &folio->lru);
+ if (nr_pages >= batch)
+ list_cut_before(&folios, from, &folio2->lru);
else
list_splice_init(from, &folios);
rc = migrate_pages_batch(&folios, get_new_page, put_new_page, private,
--
2.39.2

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