linux-kernel - [PATCH v3 03/15] mm,madvise: call soft_offline_page() without MF_COUNT

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Date:   Wed, 24 Jun 2020 15:01:25 +0000
From:   nao.horiguchi@...il.com
To:     linux-mm@...ck.org
Cc:     mhocko@...nel.org, akpm@...ux-foundation.org,
        mike.kravetz@...cle.com, osalvador@...e.de, tony.luck@...el.com,
        david@...hat.com, aneesh.kumar@...ux.vnet.ibm.com,
        zeil@...dex-team.ru, naoya.horiguchi@....com,
        linux-kernel@...r.kernel.org
Subject: [PATCH v3 03/15] mm,madvise: call soft_offline_page() without MF_COUNT_INCREASED

From: Naoya Horiguchi <naoya.horiguchi@....com>

The call to get_user_pages_fast is only to get the pointer to a struct
page of a given address, pinning it is memory-poisoning handler's job,
so drop the refcount grabbed by get_user_pages_fast().

Note that the target page is still pinned after this put_page() because
the current process should have refcount from mapping.

Signed-off-by: Naoya Horiguchi <naoya.horiguchi@....com>
Signed-off-by: Oscar Salvador <osalvador@...e.de>
---
 mm/madvise.c | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git v5.8-rc1-mmots-2020-06-20-21-44/mm/madvise.c v5.8-rc1-mmots-2020-06-20-21-44_patched/mm/madvise.c
index dd1d43cf026d..275b08edd428 100644
--- v5.8-rc1-mmots-2020-06-20-21-44/mm/madvise.c
+++ v5.8-rc1-mmots-2020-06-20-21-44_patched/mm/madvise.c
@@ -893,16 +893,24 @@ static int madvise_inject_error(int behavior,
 		 */
 		size = page_size(compound_head(page));
 
-		if (PageHWPoison(page)) {
-			put_page(page);
+		/*
+		 * The get_user_pages_fast() is just to get the pfn of the
+		 * given address, and the refcount has nothing to do with
+		 * what we try to test, so it should be released immediately.
+		 * This is racy but it's intended because the real hardware
+		 * errors could happen at any moment and memory error handlers
+		 * must properly handle the race.
+		 */
+		put_page(page);
+
+		if (PageHWPoison(page))
 			continue;
-		}
 
 		if (behavior == MADV_SOFT_OFFLINE) {
 			pr_info("Soft offlining pfn %#lx at process virtual address %#lx\n",
 					pfn, start);
 
-			ret = soft_offline_page(pfn, MF_COUNT_INCREASED);
+			ret = soft_offline_page(pfn, 0);
 			if (ret)
 				return ret;
 			continue;
@@ -910,14 +918,6 @@ static int madvise_inject_error(int behavior,
 
 		pr_info("Injecting memory failure for pfn %#lx at process virtual address %#lx\n",
 				pfn, start);
-
-		/*
-		 * Drop the page reference taken by get_user_pages_fast(). In
-		 * the absence of MF_COUNT_INCREASED the memory_failure()
-		 * routine is responsible for pinning the page to prevent it
-		 * from being released back to the page allocator.
-		 */
-		put_page(page);
 		ret = memory_failure(pfn, 0);
 		if (ret)
 			return ret;
-- 
2.17.1