linux-kernel - Re: [PATCH] mm, memory_hotplug: teach has_unmovable

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Date:   Mon, 5 Nov 2018 10:28:51 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Stable tree <stable@...r.kernel.org>
Subject: Re: [PATCH] mm, memory_hotplug: teach has_unmovable_pages about of
 LRU migrateable pages

On Mon 05-11-18 10:14:07, Michal Hocko wrote:
> Maybe we can add a retry for movable zone pages.

Or something like this. Ugly as hell, no question about that. I also
have to think about this some more to convince myself this will not
result in an endless loop under some situations.

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 48ceda313332..342d66eca0f3 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7779,12 +7779,16 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int count,
 	pfn = page_to_pfn(page);
 	for (found = 0, iter = 0; iter < pageblock_nr_pages; iter++) {
 		unsigned long check = pfn + iter;
+		unsigned long saved_flags;
 
 		if (!pfn_valid_within(check))
 			continue;
 
 		page = pfn_to_page(check);
 
+retry:
+		saved_flags = READ_ONCE(page->flags);
+
 		if (PageReserved(page))
 			goto unmovable;
 
@@ -7840,6 +7844,13 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int count,
 				page->mapping->a_ops->migratepage)
 			continue;
 
+		/*
+		 * We might race with the allocation of the page so retry
+		 * if flags have changed.
+		 */
+		if (saved_flags != READ_ONCE(page->flags))
+			goto retry;
+
 		/*
 		 * If there are RECLAIMABLE pages, we need to check
 		 * it.  But now, memory offline itself doesn't call
-- 
Michal Hocko
SUSE Labs