The quilt patch titled Subject: mm: vmscan: ensure kswapd is woken up if the wait queue is active has been removed from the -mm tree. Its filename was mm-vmscan-ensure-kswapd-is-woken-up-if-the-wait-queue-is-active.patch

This patch was dropped because an updated version will be issued

------------------------------------------------------ From: Seiji Nishikawa snishika@redhat.com Subject: mm: vmscan: ensure kswapd is woken up if the wait queue is active Date: Wed, 27 Nov 2024 00:06:12 +0900

Even after commit 501b26510ae3 ("vmstat: allow_direct_reclaim should use zone_page_state_snapshot"), a task may remain indefinitely stuck in throttle_direct_reclaim() while holding mm->rwsem.

__alloc_pages_nodemask try_to_free_pages throttle_direct_reclaim

This can cause numerous other tasks to wait on the same rwsem, leading to severe system hangups:

[1088963.358712] INFO: task python3:1670971 blocked for more than 120 seconds. [1088963.365653] Tainted: G OE -------- - - 4.18.0-553.el8_10.aarch64 #1 [1088963.373887] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [1088963.381862] task:python3 state:D stack:0 pid:1670971 ppid:1667117 flags:0x00800080 [1088963.381869] Call trace: [1088963.381872] __switch_to+0xd0/0x120 [1088963.381877] __schedule+0x340/0xac8 [1088963.381881] schedule+0x68/0x118 [1088963.381886] rwsem_down_read_slowpath+0x2d4/0x4b8

The issue arises when allow_direct_reclaim(pgdat) returns false, preventing progress even when the pgdat->pfmemalloc_wait wait queue is empty. Despite the wait queue being empty, the condition, allow_direct_reclaim(pgdat), may still be returning false, causing it to continue looping.

In some cases, reclaimable pages exist (zone_reclaimable_pages() returns

0), but calculations of pfmemalloc_reserve and free_pages result in

wmark_ok being false.

And then, despite the pgdat->kswapd_wait queue being non-empty, kswapd is not woken up, further exacerbating the problem:

crash> px ((struct pglist_data *) 0xffff00817fffe540)->kswapd_highest_zoneidx $775 = __MAX_NR_ZONES

The issue likely occurs under specific conditions: high memory pressure with frequent direct reclaim, contention on mmap_sem from concurrent memory allocations, reclaimable pages exist, but zone states cause wmark_ok to return false.

Modern workloads (e.g., Python multiprocessing) and changes in kernel reclaim logic may have surfaced such edge cases more prominently than before.

The workload involves concurrent Python processes under high memory pressure, leading to contention on mmap_sem. While not unusual, this workload may trigger a rare combination of conditions that expose the issue.

This patch modifies allow_direct_reclaim() to wake kswapd if the pgdat->kswapd_wait queue is active, regardless of whether wmark_ok is true or false. This change ensures kswapd does not miss wake-ups under high memory pressure, reducing the risk of task stalls in the throttled reclaim path.

Link: https://lkml.kernel.org/r/20241126150612.114561-1-snishika@redhat.com Signed-off-by: Seiji Nishikawa snishika@redhat.com Cc: Mel Gorman mgorman@techsingularity.net Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/vmscan.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/mm/vmscan.c~mm-vmscan-ensure-kswapd-is-woken-up-if-the-wait-queue-is-active +++ a/mm/vmscan.c @@ -6389,8 +6389,8 @@ static bool allow_direct_reclaim(pg_data

wmark_ok = free_pages > pfmemalloc_reserve / 2;

- /* kswapd must be awake if processes are being throttled */ - if (!wmark_ok && waitqueue_active(&pgdat->kswapd_wait)) { + /* Always wake up kswapd if the wait queue is not empty */ + if (waitqueue_active(&pgdat->kswapd_wait)) { if (READ_ONCE(pgdat->kswapd_highest_zoneidx) > ZONE_NORMAL) WRITE_ONCE(pgdat->kswapd_highest_zoneidx, ZONE_NORMAL);

_

Patches currently in -mm which might be from snishika@redhat.com are

mm-vmscan-account-for-free-pages-to-prevent-infinite-loop-in-throttle_direct_reclaim.patch