From: Michal Hocko mhocko@suse.com Subject: mm, memory_hotplug: do not back off draining pcp free pages from kworker context

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("mm, page_alloc: drain per-cpu pages from workqueue context") because the original IPI based pcp draining has been replaced by a WQ based one and the check wanted to prevent from recursion and inter workers dependencies.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.

Link: http://lkml.kernel.org/r/20170828093341.26341-1-mhocko@kernel.org Fixes: 0ccce3b924212 ("mm, page_alloc: drain per-cpu pages from workqueue context") Signed-off-by: Michal Hocko mhocko@suse.com Cc: Mel Gorman mgorman@suse.de Cc: Tejun Heo tj@kernel.org Cc: stable@vger.kernel.org [4.11+] Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

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

diff -puN mm/page_alloc.c~mm-memory_hotplug-do-not-back-off-draining-pcp-free-pages-from-kworker-context mm/page_alloc.c --- a/mm/page_alloc.c~mm-memory_hotplug-do-not-back-off-draining-pcp-free-pages-from-kworker-context +++ a/mm/page_alloc.c @@ -2507,10 +2507,6 @@ 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 _