[PATCH] mm, memory_hotplug: do not back off draining pcp free pages from kworker context

[Michal Hocko]

From: Michal Hocko <mhocko@xxxxxxxx>

drain_all_pages backs off when called from a kworker context since
0ccce3b924212 ("mm, page_alloc: drain per-cpu pages from workqueue
context") because the original IPI based pcp draining has been replaced
by a WQ based one and the check wanted to prevent from recursion and
inter workers dependencies. This has made some sense at the time
because the system WQ has been used and one worker holding the lock
could be blocked while waiting for new workers to emerge which can be a
problem under OOM conditions.

Since then ce612879ddc7 ("mm: move pcp and lru-pcp draining into single
wq") has moved draining to a dedicated (mm_percpu_wq) WQ with a rescuer
so we shouldn't depend on any other WQ activity to make a forward
progress so calling drain_all_pages from a worker context is safe as
long as this doesn't happen from mm_percpu_wq itself which is not the
case because all workers are required to _not_ depend on any MM locks.

Why is this a problem in the first place? ACPI driven memory hot-remove
(acpi_device_hotplug) is executed from the worker context. We end
up calling __offline_pages to free all the pages and that requires
both lru_add_drain_all_cpuslocked and drain_all_pages to do their job
otherwise we can have dangling pages on pcp lists and fail the offline
operation (__test_page_isolated_in_pageblock would see a page with 0
ref. count but without PageBuddy set).

Fix the issue by removing the worker check in drain_all_pages.
lru_add_drain_all_cpuslocked doesn't have this restriction so it works
as expected.

Fixes: 0ccce3b924212 ("mm, page_alloc: drain per-cpu pages from workqueue context")
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---

 mm/page_alloc.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ffead2159001..a94e1847bb0d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2477,10 +2477,6 @@ void drain_all_pages(struct zone *zone)
 	if (WARN_ON_ONCE(!mm_percpu_wq))
 		return;
 
-	/* Workqueues cannot recurse */
-	if (current->flags & PF_WQ_WORKER)
-		return;
-
 	/*
 	 * Do not drain if one is already in progress unless it's specific to
 	 * a zone. Such callers are primarily CMA and memory hotplug and need
-- 
2.13.2