Re: [PATCH v2 03/10] mm/memory_hotplug: simplify page offlining

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 19 Aug 2020 19:59:50 +0200 David Hildenbrand <david@redhat.com> wrote:

> We make sure that we cannot have any memory holes right at the beginning
> of offline_pages(). We no longer need walk_system_ram_range() and can
> call test_pages_isolated() and __offline_isolated_pages() directly.
> 
> offlined_pages always corresponds to nr_pages, so we can simplify that.

This patch ran afoul of Pavel's "mm/memory_hotplug: drain per-cpu pages
again during memory offline", here:

> @@ -1481,7 +1459,7 @@ static int count_system_ram_pages_cb(unsigned long start_pfn,
>  int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages)
>  {
>  	const unsigned long end_pfn = start_pfn + nr_pages;
> -	unsigned long pfn, system_ram_pages = 0, offlined_pages = 0;
> +	unsigned long pfn, system_ram_pages = 0;
>  	int ret, node, nr_isolate_pageblock;
>  	unsigned long flags;
>  	struct zone *zone;
> @@ -1579,16 +1557,12 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages)
>  			reason = "failure to dissolve huge pages";
>  			goto failed_removal_isolated;
>  		}
> -		/* check again */
> -		ret = walk_system_ram_range(start_pfn, end_pfn - start_pfn,
> -					    NULL, check_pages_isolated_cb);
> -	} while (ret);
> -
> -	/* Ok, all of our target is isolated.
> -	   We cannot do rollback at this point. */
> -	walk_system_ram_range(start_pfn, end_pfn - start_pfn,
> -			      &offlined_pages, offline_isolated_pages_cb);
> -	pr_info("Offlined Pages %ld\n", offlined_pages);
> +	} while (test_pages_isolated(start_pfn, end_pfn, MEMORY_OFFLINE));
> +
> +	/* Mark all sections offline and remove free pages from the buddy. */
> +	__offline_isolated_pages(start_pfn, end_pfn);
> +	pr_info("Offlined Pages %ld\n", nr_pages);
> +
>  	/*
>  	 * Onlining will reset pagetype flags and makes migrate type

I did this.  Looks OK?

From: David Hildenbrand <david@redhat.com>
Subject: mm/memory_hotplug: simplify page offlining

We make sure that we cannot have any memory holes right at the beginning
of offline_pages().  We no longer need walk_system_ram_range() and can
call test_pages_isolated() and __offline_isolated_pages() directly.

offlined_pages always corresponds to nr_pages, so we can simplify that.

Link: https://lkml.kernel.org/r/20200819175957.28465-4-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: Wei Yang <richard.weiyang@linux.alibaba.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: Charan Teja Reddy <charante@codeaurora.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Logan Gunthorpe <logang@deltatee.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michel Lespinasse <walken@google.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Tony Luck <tony.luck@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/memory_hotplug.c |   61 +++++++++++++++++-------------------------
 1 file changed, 25 insertions(+), 36 deletions(-)

--- a/mm/memory_hotplug.c~mm-memory_hotplug-simplify-page-offlining
+++ a/mm/memory_hotplug.c
@@ -1383,28 +1383,6 @@ do_migrate_range(unsigned long start_pfn
 	return ret;
 }
 
-/* Mark all sections offline and remove all free pages from the buddy. */
-static int
-offline_isolated_pages_cb(unsigned long start, unsigned long nr_pages,
-			void *data)
-{
-	unsigned long *offlined_pages = (unsigned long *)data;
-
-	*offlined_pages += __offline_isolated_pages(start, start + nr_pages);
-	return 0;
-}
-
-/*
- * Check all pages in range, recorded as memory resource, are isolated.
- */
-static int
-check_pages_isolated_cb(unsigned long start_pfn, unsigned long nr_pages,
-			void *data)
-{
-	return test_pages_isolated(start_pfn, start_pfn + nr_pages,
-				   MEMORY_OFFLINE);
-}
-
 static int __init cmdline_parse_movable_node(char *p)
 {
 	movable_node_enabled = true;
@@ -1491,7 +1469,7 @@ static int count_system_ram_pages_cb(uns
 int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages)
 {
 	const unsigned long end_pfn = start_pfn + nr_pages;
-	unsigned long pfn, system_ram_pages = 0, offlined_pages = 0;
+	unsigned long pfn, system_ram_pages = 0;
 	int ret, node, nr_isolate_pageblock;
 	unsigned long flags;
 	struct zone *zone;
@@ -1589,16 +1567,27 @@ int __ref offline_pages(unsigned long st
 			reason = "failure to dissolve huge pages";
 			goto failed_removal_isolated;
 		}
-		/* check again */
-		ret = walk_system_ram_range(start_pfn, end_pfn - start_pfn,
-					    NULL, check_pages_isolated_cb);
-	} while (ret);
-
-	/* Ok, all of our target is isolated.
-	   We cannot do rollback at this point. */
-	walk_system_ram_range(start_pfn, end_pfn - start_pfn,
-			      &offlined_pages, offline_isolated_pages_cb);
-	pr_info("Offlined Pages %ld\n", offlined_pages);
+
+		/*
+		 * 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.
+		 *
+		 * 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.
+		 */
+		if (ret)
+			drain_all_pages(zone);
+	} while (test_pages_isolated(start_pfn, end_pfn, MEMORY_OFFLINE));
+
+	/* Mark all sections offline and remove free pages from the buddy. */
+	__offline_isolated_pages(start_pfn, end_pfn);
+	pr_info("Offlined Pages %ld\n", nr_pages);
+
 	/*
 	 * Onlining will reset pagetype flags and makes migrate type
 	 * MOVABLE, so just need to decrease the number of isolated
@@ -1609,11 +1598,11 @@ int __ref offline_pages(unsigned long st
 	spin_unlock_irqrestore(&zone->lock, flags);
 
 	/* removal success */
-	adjust_managed_page_count(pfn_to_page(start_pfn), -offlined_pages);
-	zone->present_pages -= offlined_pages;
+	adjust_managed_page_count(pfn_to_page(start_pfn), -nr_pages);
+	zone->present_pages -= nr_pages;
 
 	pgdat_resize_lock(zone->zone_pgdat, &flags);
-	zone->zone_pgdat->node_present_pages -= offlined_pages;
+	zone->zone_pgdat->node_present_pages -= nr_pages;
 	pgdat_resize_unlock(zone->zone_pgdat, &flags);
 
 	init_per_zone_wmark_min();
_