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

From: Hugh Dickins
Date: Tue Feb 28 2023 - 01:13:39 EST

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On Fri, 24 Feb 2023, Huang Ying wrote:

> 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/
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Reported-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Reported-by: "Xu, Pengfei" <pengfei.xu@xxxxxxxxx>
> 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: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> ---
> mm/migrate.c | 62 ++++++++++++++++------------------------------------
> 1 file changed, 19 insertions(+), 43 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 37865f85df6d..7ac37dbbf307 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1106,7 +1106,7 @@ 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)
> {
> @@ -1157,17 +1157,6 @@ 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;
> @@ -1247,7 +1236,7 @@ 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);

Why that change, I wonder? The TTU_BATCH_FLUSH can still be useful for
gathering multiple cross-CPU TLB flushes into one, even when it's only
a single page in the batch.

> page_was_mapped = 1;
> }
>
> @@ -1261,7 +1250,7 @@ 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);
> @@ -1634,11 +1623,9 @@ 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;
>
> retry:
> rc_saved = 0;
> - avoid_force_lock = false;
> retry = 1;
> for (pass = 0;
> pass < NR_MAX_MIGRATE_PAGES_RETRY && (retry || large_retry);
> @@ -1683,15 +1670,14 @@ 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
> */
> @@ -1743,14 +1729,6 @@ 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++;
> @@ -1765,11 +1743,6 @@ 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;
> @@ -1894,17 +1867,15 @@ 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;

It goes away in a later patch anyway, but I didn't understand that change -
I thought this was a point at which no locks are held. Oh, perhaps I get
it now: because the batch of 1 is here becoming a batch of HPAGE_PMD_NR?

> 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;
> }
>
> @@ -1939,7 +1910,7 @@ 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);
> @@ -1953,6 +1924,11 @@ 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) {
> @@ -1963,11 +1939,11 @@ 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;

Yes, the off-by-one fixes look good.

> }
> - 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,
> --
> 2.39.1

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In reply to: Huang Ying: "[PATCH 1/3] migrate_pages: fix deadlock in batched migration"
Next in thread: Huang, Ying: "Re: [PATCH 1/3] migrate_pages: fix deadlock in batched migration"
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