linux-kernel - [PATCH] mm/memory_hotplug: drain per-cpu pages again during memory offline

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Date:   Tue,  1 Sep 2020 08:46:15 -0400
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        mhocko@...e.com, linux-mm@...ck.org, pasha.tatashin@...een.com
Subject: [PATCH] mm/memory_hotplug: drain per-cpu pages again during memory offline

There is a race during page offline that can lead to infinite loop:
a page never ends up on a buddy list and __offline_pages() keeps
retrying infinitely or until a termination signal is received.

Thread#1 - a new process:

load_elf_binary
 begin_new_exec
  exec_mmap
   mmput
    exit_mmap
     tlb_finish_mmu
      tlb_flush_mmu
       release_pages
        free_unref_page_list
         free_unref_page_prepare
          set_pcppage_migratetype(page, migratetype);
             // Set page->index migration type below  MIGRATE_PCPTYPES

Thread#2 - hot-removes memory
__offline_pages
  start_isolate_page_range
    set_migratetype_isolate
      set_pageblock_migratetype(page, MIGRATE_ISOLATE);
        Set migration type to MIGRATE_ISOLATE-> set
        drain_all_pages(zone);
             // drain per-cpu page lists to buddy allocator.

Thread#1 - continue
         free_unref_page_commit
           migratetype = get_pcppage_migratetype(page);
              // get old migration type
           list_add(&page->lru, &pcp->lists[migratetype]);
              // add new page to already drained pcp list

Thread#2
Never drains pcp again, and therefore gets stuck in the loop.

The fix is to try to drain per-cpu lists again after
check_pages_isolated_cb() fails.

Signed-off-by: Pavel Tatashin <pasha.tatashin@...een.com>
Cc: stable@...r.kernel.org
---
 mm/memory_hotplug.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index e9d5ab5d3ca0..d6d54922bfce 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1575,6 +1575,15 @@ static int __ref __offline_pages(unsigned long start_pfn,
 		/* check again */
 		ret = walk_system_ram_range(start_pfn, end_pfn - start_pfn,
 					    NULL, check_pages_isolated_cb);
+		/*
+		 * per-cpu pages are drained in start_isolate_page_range, but if
+		 * there are still pages that are not free, make sure that we
+		 * drain again, because when we isolated range we might
+		 * have raced with another thread that was adding pages to
+		 * pcp list.
+		 */
+		if (ret)
+			drain_all_pages(zone);
 	} while (ret);
 
 	/* Ok, all of our target is isolated.
-- 
2.25.1