mm, page_alloc: disable pcplists during memory offline

Memory offlining relies on page isolation to guarantee a forward progress
because pages cannot be reused while they are isolated.  But the page
isolation itself doesn't prevent from races while freed pages are stored
on pcp lists and thus can be reused.  This can be worked around by
repeated draining of pcplists, as done by commit 968318261221
("mm/memory_hotplug: drain per-cpu pages again during memory offline").

David and Michal would prefer that this race was closed in a way that
callers of page isolation who need stronger guarantees don't need to
repeatedly drain.  David suggested disabling pcplists usage completely
during page isolation, instead of repeatedly draining them.

To achieve this without adding special cases in alloc/free fastpath, we
can use the same approach as boot pagesets - when pcp->high is 0, any
pcplist addition will be immediately flushed.

The race can thus be closed by setting pcp->high to 0 and draining
pcplists once, before calling start_isolate_page_range().  The draining
will serialize after processes that already disabled interrupts and read
the old value of pcp->high in free_unref_page_commit(), and processes that
have not yet disabled interrupts, will observe pcp->high == 0 when they
are rescheduled, and skip pcplists.  This guarantees no stray pages on
pcplists in zones where isolation happens.

This patch thus adds zone_pcp_disable() and zone_pcp_enable() functions
that page isolation users can call before start_isolate_page_range() and
after unisolating (or offlining) the isolated pages.

Also, drain_all_pages() is optimized to only execute on cpus where
pcplists are not empty.  The check can however race with a free to pcplist
that has not yet increased the pcp->count from 0 to 1.  Thus make the
drain optionally skip the racy check and drain on all cpus, and use this
option in zone_pcp_disable().

As we have to avoid external updates to high and batch while pcplists are
disabled, we take pcp_batch_high_lock in zone_pcp_disable() and release it
in zone_pcp_enable().  This also synchronizes multiple users of
zone_pcp_disable()/enable().

Currently the only user of this functionality is offline_pages().

[vbabka@suse.cz: add comment, per David]
  Link: https://lkml.kernel.org/r/527480ef-ed72-e1c1-52a0-1c5b0113df45@suse.cz

Link: https://lkml.kernel.org/r/20201111092812.11329-8-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Suggested-by: David Hildenbrand <david@redhat.com>
Suggested-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 16 deletions

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 3c494ab0d075..e0a561c550b3 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1491,17 +1491,21 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages)
 	}
 	node = zone_to_nid(zone);
 
+	/*
+	 * Disable pcplists so that page isolation cannot race with freeing
+	 * in a way that pages from isolated pageblock are left on pcplists.
+	 */
+	zone_pcp_disable(zone);
+
 	/* set above range as isolated */
 	ret = start_isolate_page_range(start_pfn, end_pfn,
 				       MIGRATE_MOVABLE,
 				       MEMORY_OFFLINE | REPORT_FAILURE);
 	if (ret) {
 		reason = "failure to isolate range";
-		goto failed_removal;
+		goto failed_removal_pcplists_disabled;
 	}
 
-	drain_all_pages(zone);
-
 	arg.start_pfn = start_pfn;
 	arg.nr_pages = nr_pages;
 	node_states_check_changes_offline(nr_pages, zone, &arg);
@@ -1551,20 +1555,8 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages)
 			goto failed_removal_isolated;
 		}
 
-		/*
-		 * per-cpu pages are drained after 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.
-		 *
-		 * Forward progress should be still guaranteed because
-		 * pages on the pcp list can only belong to MOVABLE_ZONE
-		 * because has_unmovable_pages explicitly checks for
-		 * PageBuddy on freed pages on other zones.
-		 */
 		ret = test_pages_isolated(start_pfn, end_pfn, MEMORY_OFFLINE);
-		if (ret)
-			drain_all_pages(zone);
+
 	} while (ret);
 
 	/* Mark all sections offline and remove free pages from the buddy. */
@@ -1580,6 +1572,8 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages)
 	zone->nr_isolate_pageblock -= nr_pages / pageblock_nr_pages;
 	spin_unlock_irqrestore(&zone->lock, flags);
 
+	zone_pcp_enable(zone);
+
 	/* removal success */
 	adjust_managed_page_count(pfn_to_page(start_pfn), -nr_pages);
 	zone->present_pages -= nr_pages;
@@ -1612,6 +1606,8 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages)
 failed_removal_isolated:
 	undo_isolate_page_range(start_pfn, end_pfn, MIGRATE_MOVABLE);
 	memory_notify(MEM_CANCEL_OFFLINE, &arg);
+failed_removal_pcplists_disabled:
+	zone_pcp_enable(zone);
 failed_removal:
 	pr_debug("memory offlining [mem %#010llx-%#010llx] failed due to %s\n",
 		 (unsigned long long) start_pfn << PAGE_SHIFT,