Hi Ying,
On 2024/7/29 09:38, Huang, Ying wrote:
Hi, Xiang,
Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx> writes:
Currently, migrate_pages_batch() can lock multiple locked folios
with an arbitrary order. Although folio_trylock() is used to avoid
deadlock as commit 2ef7dbb26990 ("migrate_pages: try migrate in batch
asynchronously firstly") mentioned, it seems try_split_folio() is
still missing.
It was found by compaction stress test when I explicitly enable EROFS
compressed files to use large folios, which case I cannot reproduce with
the same workload if large folio support is off (current mainline).
Typically, filesystem reads (with locked file-backed folios) could use
another bdev/meta inode to load some other I/Os (e.g. inode extent
metadata or caching compressed data), so the locking order will be:
file-backed folios (A)
bdev/meta folios (B)
The following calltrace shows the deadlock:
Thread 1 takes (B) lock and tries to take folio (A) lock
Thread 2 takes (A) lock and tries to take folio (B) lock
[Thread 1]
INFO: task stress:1824 blocked for more than 30 seconds.
Tainted: G OE 6.10.0-rc7+ #6
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:stress state:D stack:0 pid:1824 tgid:1824 ppid:1822 flags:0x0000000c
Call trace:
__switch_to+0xec/0x138
__schedule+0x43c/0xcb0
schedule+0x54/0x198
io_schedule+0x44/0x70
folio_wait_bit_common+0x184/0x3f8
<-- folio mapping ffff00036d69cb18 index 996 (**)
__folio_lock+0x24/0x38
migrate_pages_batch+0x77c/0xea0 // try_split_folio (mm/migrate.c:1486:2)
// migrate_pages_batch (mm/migrate.c:1734:16)
<--- LIST_HEAD(unmap_folios) has
..
folio mapping 0xffff0000d184f1d8 index 1711; (*)
folio mapping 0xffff0000d184f1d8 index 1712;
..
migrate_pages+0xb28/0xe90
compact_zone+0xa08/0x10f0
compact_node+0x9c/0x180
sysctl_compaction_handler+0x8c/0x118
proc_sys_call_handler+0x1a8/0x280
proc_sys_write+0x1c/0x30
vfs_write+0x240/0x380
ksys_write+0x78/0x118
__arm64_sys_write+0x24/0x38
invoke_syscall+0x78/0x108
el0_svc_common.constprop.0+0x48/0xf0
do_el0_svc+0x24/0x38
el0_svc+0x3c/0x148
el0t_64_sync_handler+0x100/0x130
el0t_64_sync+0x190/0x198
[Thread 2]
INFO: task stress:1825 blocked for more than 30 seconds.
Tainted: G OE 6.10.0-rc7+ #6
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:stress state:D stack:0 pid:1825 tgid:1825 ppid:1822 flags:0x0000000c
Call trace:
__switch_to+0xec/0x138
__schedule+0x43c/0xcb0
schedule+0x54/0x198
io_schedule+0x44/0x70
folio_wait_bit_common+0x184/0x3f8
<-- folio = 0xfffffdffc6b503c0 (mapping == 0xffff0000d184f1d8 index == 1711) (*)
__folio_lock+0x24/0x38
z_erofs_runqueue+0x384/0x9c0 [erofs]
z_erofs_readahead+0x21c/0x350 [erofs] <-- folio mapping 0xffff00036d69cb18 range from [992, 1024] (**)
read_pages+0x74/0x328
page_cache_ra_order+0x26c/0x348
ondemand_readahead+0x1c0/0x3a0
page_cache_sync_ra+0x9c/0xc0
filemap_get_pages+0xc4/0x708
filemap_read+0x104/0x3a8
generic_file_read_iter+0x4c/0x150
vfs_read+0x27c/0x330
ksys_pread64+0x84/0xd0
__arm64_sys_pread64+0x28/0x40
invoke_syscall+0x78/0x108
el0_svc_common.constprop.0+0x48/0xf0
do_el0_svc+0x24/0x38
el0_svc+0x3c/0x148
el0t_64_sync_handler+0x100/0x130
el0t_64_sync+0x190/0x198
Fixes: 5dfab109d519 ("migrate_pages: batch _unmap and _move")
Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>
---
mm/migrate.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/mm/migrate.c b/mm/migrate.c
index 20cb9f5f7446..a912e4b83228 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1483,7 +1483,8 @@ static inline int try_split_folio(struct folio *folio, struct list_head *split_f
{
int rc;
- folio_lock(folio);
+ if (!folio_trylock(folio))
+ return -EAGAIN;
rc = split_folio_to_list(folio, split_folios);
folio_unlock(folio);
if (!rc)
Good catch! Thanks for the fixing!
The deadlock is similar as the one we fixed in commit fb3592c41a44
("migrate_pages: fix deadlock in batched migration"). But apparently,
we missed this case.
For the fix, I think that we should still respect migrate_mode because
users may prefer migration success over blocking.
@@ -1492,11 +1492,17 @@ static int unmap_and_move_huge_page(new_folio_t get_new_folio,
return rc;
}
-static inline int try_split_folio(struct folio *folio, struct list_head *split_folios)
+static inline int try_split_folio(struct folio *folio, struct list_head *split_folios,
+ enum migrate_mode mode)
{
int rc;
- folio_lock(folio);
+ if (mode == MIGRATE_ASYNC) {
+ if (!folio_trylock(folio))
+ return -EAGAIN;
+ } else {
+ folio_lock(folio);
+ }
rc = split_folio_to_list(folio, split_folios);
folio_unlock(folio);
if (!rc)
Okay, yeah it looks better since it seems I missed the fallback
part in migrate_pages_sync().
Let me send the next version to follow your advice, thanks.
Thanks,
Gao Xiang
--
Best Regards,
Huang, Ying
