linux-kernel - [PATCH 3/7] mm,madvise: call soft_offline_page() without MF_COUNT

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Date:   Thu, 19 Nov 2020 11:57:12 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     akpm@...ux-foundation.org
Cc:     n-horiguchi@...jp.nec.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Oscar Salvador <osalvador@...e.de>
Subject: [PATCH 3/7] 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 | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index c6b5524add58..7a0f64b93635 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -900,20 +900,23 @@ static int madvise_inject_error(int behavior,
 		 */
 		size = page_size(compound_head(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 (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);
 		} else {
 			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);
 		}
 
-- 
2.26.2