mm/vmscan.c: fix potential deadlock in reclaim_pages()

Theoretically without the protect from memalloc_noreclaim_save() and
memalloc_noreclaim_restore(), reclaim_pages() can go into the block
I/O layer recursively and deadlock.

Querying 'reclaim_pages' in our kernel crash databases didn't yield
any results. So the deadlock seems unlikely to happen. A possible
explanation is that the only user of reclaim_pages(), i.e.,
MADV_PAGEOUT, is usually called before memory pressure builds up,
e.g., on Android and Chrome OS. Under such a condition, allocations in
the block I/O layer can be fulfilled without diverting to direct
reclaim and therefore the recursion is avoided.

Link: https://lkml.kernel.org/r/20210622074642.785473-1-yuzhao@google.com
Link: https://lkml.kernel.org/r/20210614194727.2684053-1-yuzhao@google.com
Signed-off-by: Yu Zhao <yuzhao@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index d7c3cb8..7b52ab1 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1701,6 +1701,7 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
        unsigned int nr_reclaimed;
        struct page *page, *next;
        LIST_HEAD(clean_pages);
+       unsigned int noreclaim_flag;
 
        list_for_each_entry_safe(page, next, page_list, lru) {
                if (!PageHuge(page) && page_is_file_lru(page) &&
@@ -1711,8 +1712,17 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
                }
        }
 
+       /*
+        * We should be safe here since we are only dealing with file pages and
+        * we are not kswapd and therefore cannot write dirty file pages. But
+        * call memalloc_noreclaim_save() anyway, just in case these conditions
+        * change in the future.
+        */
+       noreclaim_flag = memalloc_noreclaim_save();
        nr_reclaimed = shrink_page_list(&clean_pages, zone->zone_pgdat, &sc,
                                        &stat, true);
+       memalloc_noreclaim_restore(noreclaim_flag);
+
        list_splice(&clean_pages, page_list);
        mod_node_page_state(zone->zone_pgdat, NR_ISOLATED_FILE,
                            -(long)nr_reclaimed);
@@ -2306,6 +2316,7 @@ unsigned long reclaim_pages(struct list_head *page_list)
        LIST_HEAD(node_page_list);
        struct reclaim_stat dummy_stat;
        struct page *page;
+       unsigned int noreclaim_flag;
        struct scan_control sc = {
                .gfp_mask = GFP_KERNEL,
                .priority = DEF_PRIORITY,
@@ -2314,6 +2325,8 @@ unsigned long reclaim_pages(struct list_head *page_list)
                .may_swap = 1,
        };
 
+       noreclaim_flag = memalloc_noreclaim_save();
+
        while (!list_empty(page_list)) {
                page = lru_to_page(page_list);
                if (nid == NUMA_NO_NODE) {
@@ -2350,6 +2363,8 @@ unsigned long reclaim_pages(struct list_head *page_list)
                }
        }
 
+       memalloc_noreclaim_restore(noreclaim_flag);
+
        return nr_reclaimed;
 }