Re: [RFC] mm/migrate: make sure folio_unlock() before folio_wait_writeback()

Yeoreum Yun
2025-10-02T11:43:36Z

Hi Byoungchul,

> DEPT(Dependency Tracker) reported a deadlock:
>
> ===================================================
> DEPT: Circular dependency has been detected.
> 6.15.11-00046-g2c223fa7bd9a-dirty #13 Not tainted
> ---------------------------------------------------
> summary
> ---------------------------------------------------
> *** DEADLOCK ***
>
> context A
> [S] (unknown)(pg_locked_map:0)
> [W] dept_page_wait_on_bit(pg_writeback_map:0)
> [E] dept_page_clear_bit(pg_locked_map:0)
>
> context B
> [S] (unknown)(pg_writeback_map:0)
> [W] dept_page_wait_on_bit(pg_locked_map:0)
> [E] dept_page_clear_bit(pg_writeback_map:0)
>
> [S]: start of the event context
> [W]: the wait blocked
> [E]: the event not reachable
> ---------------------------------------------------
> context A's detail
> ---------------------------------------------------
> context A
> [S] (unknown)(pg_locked_map:0)
> [W] dept_page_wait_on_bit(pg_writeback_map:0)
> [E] dept_page_clear_bit(pg_locked_map:0)
>
> [S] (unknown)(pg_locked_map:0):
> (N/A)
>
> [W] dept_page_wait_on_bit(pg_writeback_map:0):
> [<ffff800080589c94>] folio_wait_bit+0x2c/0x38
> stacktrace:
> folio_wait_bit_common+0x824/0x8b8
> folio_wait_bit+0x2c/0x38
> folio_wait_writeback+0x5c/0xa4
> migrate_pages_batch+0x5e4/0x1788
> migrate_pages+0x15c4/0x1840
> compact_zone+0x9c8/0x1d20
> compact_node+0xd4/0x27c
> sysctl_compaction_handler+0x104/0x194
> proc_sys_call_handler+0x25c/0x3f8
> proc_sys_write+0x20/0x2c
> do_iter_readv_writev+0x350/0x448
> vfs_writev+0x1ac/0x44c
> do_pwritev+0x100/0x15c
> __arm64_sys_pwritev2+0x6c/0xcc
> invoke_syscall.constprop.0+0x64/0x18c
> el0_svc_common.constprop.0+0x80/0x198
>
> [E] dept_page_clear_bit(pg_locked_map:0):
> [<ffff800080700914>] migrate_folio_undo_src+0x1b4/0x200
> stacktrace:
> migrate_folio_undo_src+0x1b4/0x200
> migrate_pages_batch+0x1578/0x1788
> migrate_pages+0x15c4/0x1840
> compact_zone+0x9c8/0x1d20
> compact_node+0xd4/0x27c
> sysctl_compaction_handler+0x104/0x194
> proc_sys_call_handler+0x25c/0x3f8
> proc_sys_write+0x20/0x2c
> do_iter_readv_writev+0x350/0x448
> vfs_writev+0x1ac/0x44c
> do_pwritev+0x100/0x15c
> __arm64_sys_pwritev2+0x6c/0xcc
> invoke_syscall.constprop.0+0x64/0x18c
> el0_svc_common.constprop.0+0x80/0x198
> do_el0_svc+0x28/0x3c
> el0_svc+0x50/0x220
> ---------------------------------------------------
> context B's detail
> ---------------------------------------------------
> context B
> [S] (unknown)(pg_writeback_map:0)
> [W] dept_page_wait_on_bit(pg_locked_map:0)
> [E] dept_page_clear_bit(pg_writeback_map:0)
>
> [S] (unknown)(pg_writeback_map:0):
> (N/A)
>
> [W] dept_page_wait_on_bit(pg_locked_map:0):
> [<ffff80008081e478>] bdev_getblk+0x58/0x120
> stacktrace:
> find_get_block_common+0x224/0xbc4
> bdev_getblk+0x58/0x120
> __ext4_get_inode_loc+0x194/0x98c
> ext4_get_inode_loc+0x4c/0xcc
> ext4_reserve_inode_write+0x74/0x158
> __ext4_mark_inode_dirty+0xd4/0x4e0
> __ext4_ext_dirty+0x118/0x164
> ext4_ext_map_blocks+0x1578/0x2ca8
> ext4_map_blocks+0x2a4/0xa60
> ext4_convert_unwritten_extents+0x1b0/0x3c0
> ext4_convert_unwritten_io_end_vec+0x90/0x1a0
> ext4_end_io_end+0x58/0x194
> ext4_end_io_rsv_work+0xc4/0x150
> process_one_work+0x3b4/0xac0
> worker_thread+0x2b0/0x53c
> kthread+0x1a0/0x33c
>
> [E] dept_page_clear_bit(pg_writeback_map:0):
> [<ffff8000809dfc5c>] ext4_finish_bio+0x638/0x820
> stacktrace:
> folio_end_writeback+0x140/0x488
> ext4_finish_bio+0x638/0x820
> ext4_release_io_end+0x74/0x188
> ext4_end_io_end+0xa0/0x194
> ext4_end_io_rsv_work+0xc4/0x150
> process_one_work+0x3b4/0xac0
> worker_thread+0x2b0/0x53c
> kthread+0x1a0/0x33c
> ret_from_fork+0x10/0x20
>
> To simplify the scenario:
>
> context X (wq worker) context Y (process context)
>
> migrate_pages_batch()
> ext4_end_io_end() ...
> ... migrate_folio_unmap()
> ext4_get_inode_loc() ...
> ... folio_lock() // hold the folio lock
> bdev_getblk() ...
> ... folio_wait_writeback() // wait forever
> __find_get_block_slow()
> ... ...
> folio_lock() // wait forever
> folio_unlock() migrate_folio_undo_src()
> ...
> ... folio_unlock() // never reachable
> ext4_finish_bio()
> ...
> folio_end_writeback() // never reachable
>
> context X is waiting for the folio lock to be released by context Y,
> while context Y is waiting for the writeback to end in context X.
> Ultimately, two contexts are waiting for the event that will never
> happen, say, deadlock.
>
> *Only one* of the following two conditions should be allowed, or we
> cannot avoid this kind of deadlock:
>
> 1. while holding a folio lock (and heading for folio_unlock()),
> waiting for a writeback to end,
> 2. while heading for the writeback end, waiting for the folio lock to
> be released,
>
> Since allowing 2 and avoiding 1 sound more sensible than the other,
> remove the first condition by making sure folio_unlock() before
> folio_wait_writeback() in migrate_folio_unmap().
>
> Fixes: 49d2e9cc45443 ("[PATCH] Swap Migration V5: migrate_pages() function")
> Reported-by: Yunseong Kim <ysk@kzalloc.com>
> Signed-off-by: Byungchul Park <byungchul@sk.com>
> Tested-by: Yunseong Kim <ysk@kzalloc.com>
> ---
>
> Hi,
>
> Thanks to Yunseong for reporting the issue, testing, and confirming if
> this patch can resolve the issue. We used the latest version of DEPT
> to detect the issue:
>
> https://lore.kernel.org/all/20251002081247.51255-1-byungchul@sk.com/
>
> I mentioned in the commit message above like:
>
> *Only one* of the following two conditions should be allowed, or we
> cannot avoid this kind of deadlock:
>
> 1. while holding a folio lock (and heading for folio_unlock()),
> waiting for a writeback to end,
> 2. while heading for the writeback end, waiting for the folio lock
> to be released,
>
> Honestly, I'm not convinced which one we should choose between two, I
> chose 'allowing 2 and avoiding 1' to resolve this issue though.
>
> However, please let me know if I was wrong and we should go for
> 'allowing 1 and avoiding 2'. If so, I should try a different approach,
> for example, to fix by preventing folio_lock() or using folio_try_lock()
> while heading for writeback end in ext4_end_io_end() or something.
>
> To Yunseong,
>
> The link you shared for a system hang is:
>
> https://gist.github.com/kzall0c/a6091bb2fd536865ca9aabfd017a1fc5
>
> I think an important stacktrace for this issue, this is, waiting for
> PG_writeback, was missed in the log.
>
> Byungchul
>
> ---
> mm/migrate.c | 57 +++++++++++++++++++++++++++++++++++++---------------
> 1 file changed, 41 insertions(+), 16 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 9e5ef39ce73a..60b0b054f27a 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1215,6 +1215,17 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
>
> dst->private = NULL;
>
> +retry_wait_writeback:
> + /*
> + * Only in the case of a full synchronous migration is it
> + * necessary to wait for PageWriteback. In the async case, the
> + * retry loop is too short and in the sync-light case, the
> + * overhead of stalling is too much. Plus, do not write-back if
> + * it's in the middle of direct compaction
> + */
> + if (folio_test_writeback(src) && mode == MIGRATE_SYNC)
> + folio_wait_writeback(src);
> +
> if (!folio_trylock(src)) {
> if (mode == MIGRATE_ASYNC)
> goto out;
> @@ -1245,27 +1256,41 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
>
> folio_lock(src);
> }
> - locked = true;
> - if (folio_test_mlocked(src))
> - old_page_state |= PAGE_WAS_MLOCKED;
>
> if (folio_test_writeback(src)) {
> - /*
> - * Only in the case of a full synchronous migration is it
> - * necessary to wait for PageWriteback. In the async case,
> - * the retry loop is too short and in the sync-light case,
> - * the overhead of stalling is too much
> - */
> - switch (mode) {
> - case MIGRATE_SYNC:
> - break;
> - default:
> - rc = -EBUSY;
> - goto out;
> + if (mode == MIGRATE_SYNC) {
> + /*
> + * folio_unlock() is required before trying
> + * folio_wait_writeback(). Or it leads a
> + * deadlock like:
> + *
> + * context x context y
> + * in XXX_io_end() in migrate_folio_unmap()
> + *
> + * ... ...
> + * bdev_getblk(); folio_lock();
> + *
> + * // wait forever // wait forever
> + * folio_lock(); folio_wait_writeback();
> + *
> + * ... ...
> + * folio_unlock();
> + * ... // never reachable
> + * folio_unlock();
> + * // never reachable
> + * folio_end_writeback();
> + */
> + folio_unlock(src);
> + goto retry_wait_writeback;
> }
> - folio_wait_writeback(src);
> + rc = -EBUSY;
> + goto out;
> }
>
> + locked = true;
> + if (folio_test_mlocked(src))
> + old_page_state |= PAGE_WAS_MLOCKED;
> +
> /*
> * By try_to_migrate(), src->mapcount goes down to 0 here. In this case,
> * we cannot notice that anon_vma is freed while we migrate a page.

Hmm, I still have concerns about this change.
(1) seems to imply that the use of WB_SYNC_ALL by
mpage_writebacks() is also incorrect. In addition,
this change could introduce another theoretical livelock
when the folio enters writeback frequently.

AFAIK, while a folio is under writeback,
its related buffers won’t be freed by migration, and
since try_free_buffer() checks the writeback state first,
taking folio_lock() shouldn’t be necessary while bdev_getblk().

Therefore, it seems sufficient to check whether
the folio is under writeback in __find_get_block_slow(), e.g.:

diff --git a/fs/buffer.c b/fs/buffer.c
index 6a8752f7bbed..804d33df6b0f 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -194,6 +194,9 @@ __find_get_block_slow(struct block_device *bdev, sector_t block, bool atomic)
if (IS_ERR(folio))
goto out;

+ if (folio_test_writeback(folio))
+ return true;
+
/*
* Folio lock protects the buffers. Callers that cannot block
* will fallback to serializing vs try_to_free_buffers() via

Am I missing something?

--
Sincerely,
Yeoreum Yun