[PATCH v2 4/5] mm: check __PG_HWPOISON separately from PAGE_FLAGS_CHECK_AT_*

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

The race condition addressed in commit add05cecef80 ("mm: soft-offline: don't
free target page in successful page migration") was not closed completely,
because that can happen not only for soft-offline, but also for hard-offline.
Consider that a slab page is about to be freed into buddy pool, and then an
uncorrected memory error hits the page just after entering __free_one_page(),
then VM_BUG_ON_PAGE(page->flags & PAGE_FLAGS_CHECK_AT_PREP) is triggered,
despite the fact that it's not necessary because the data on the affected
page is not consumed.

To solve it, this patch drops __PG_HWPOISON from page flag checks at
allocation/free time. I think it's justified because __PG_HWPOISON flags is
defined to prevent the page from being reused, and setting it outside the
page's alloc-free cycle is a designed behavior (not a bug.)

For recent months, I was annoyed about BUG_ON when soft-offlined page remains
on lru cache list for a while, which is avoided by calling put_page() instead
of putback_lru_page() in page migration's success path. This means that this
patch reverts a major change from commit add05cecef80 about the new refcounting
rule of soft-offlined pages, so "reuse window" revives. This will be closed
by a subsequent patch.

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
---
v1 -> v2:
- call put_page() when migration succeeded in reason == MR_MEMORY_FAILURE
- refrain from reviving "reuse prevention" by MIGRATE_ISOLATE flag
---
 include/linux/page-flags.h | 10 +++++++---
 mm/huge_memory.c           |  7 +------
 mm/migrate.c               |  5 ++++-
 mm/page_alloc.c            |  4 ++++
 4 files changed, 16 insertions(+), 10 deletions(-)

diff --git v4.2-rc4.orig/include/linux/page-flags.h v4.2-rc4/include/linux/page-flags.h
index f34e040b34e9..41c93844fb1d 100644
--- v4.2-rc4.orig/include/linux/page-flags.h
+++ v4.2-rc4/include/linux/page-flags.h
@@ -631,15 +631,19 @@ static inline void ClearPageSlabPfmemalloc(struct page *page)
 	 1 << PG_private | 1 << PG_private_2 | \
 	 1 << PG_writeback | 1 << PG_reserved | \
 	 1 << PG_slab	 | 1 << PG_swapcache | 1 << PG_active | \
-	 1 << PG_unevictable | __PG_MLOCKED | __PG_HWPOISON | \
+	 1 << PG_unevictable | __PG_MLOCKED | \
 	 __PG_COMPOUND_LOCK)
 
 /*
  * Flags checked when a page is prepped for return by the page allocator.
- * Pages being prepped should not have any flags set.  It they are set,
+ * Pages being prepped should not have these flags set.  It they are set,
  * there has been a kernel bug or struct page corruption.
+ *
+ * __PG_HWPOISON is exceptional because it needs to be kept beyond page's
+ * alloc-free cycle to prevent from reusing the page.
  */
-#define PAGE_FLAGS_CHECK_AT_PREP	((1 << NR_PAGEFLAGS) - 1)
+#define PAGE_FLAGS_CHECK_AT_PREP	\
+	(((1 << NR_PAGEFLAGS) - 1) & ~__PG_HWPOISON)
 
 #define PAGE_FLAGS_PRIVATE				\
 	(1 << PG_private | 1 << PG_private_2)
diff --git v4.2-rc4.orig/mm/huge_memory.c v4.2-rc4/mm/huge_memory.c
index c107094f79ba..097c7a4bfbd9 100644
--- v4.2-rc4.orig/mm/huge_memory.c
+++ v4.2-rc4/mm/huge_memory.c
@@ -1676,12 +1676,7 @@ static void __split_huge_page_refcount(struct page *page,
 		/* after clearing PageTail the gup refcount can be released */
 		smp_mb__after_atomic();
 
-		/*
-		 * retain hwpoison flag of the poisoned tail page:
-		 *   fix for the unsuitable process killed on Guest Machine(KVM)
-		 *   by the memory-failure.
-		 */
-		page_tail->flags &= ~PAGE_FLAGS_CHECK_AT_PREP | __PG_HWPOISON;
+		page_tail->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
 		page_tail->flags |= (page->flags &
 				     ((1L << PG_referenced) |
 				      (1L << PG_swapbacked) |
diff --git v4.2-rc4.orig/mm/migrate.c v4.2-rc4/mm/migrate.c
index ee401e4e5ef1..f2415be7d93b 100644
--- v4.2-rc4.orig/mm/migrate.c
+++ v4.2-rc4/mm/migrate.c
@@ -950,7 +950,10 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
 		list_del(&page->lru);
 		dec_zone_page_state(page, NR_ISOLATED_ANON +
 				page_is_file_cache(page));
-		if (reason != MR_MEMORY_FAILURE)
+		/* Soft-offlined page shouldn't go through lru cache list */
+		if (reason == MR_MEMORY_FAILURE)
+			put_page(page);
+		else
 			putback_lru_page(page);
 	}
 
diff --git v4.2-rc4.orig/mm/page_alloc.c v4.2-rc4/mm/page_alloc.c
index ef19f22b2b7d..775c254648f7 100644
--- v4.2-rc4.orig/mm/page_alloc.c
+++ v4.2-rc4/mm/page_alloc.c
@@ -1285,6 +1285,10 @@ static inline int check_new_page(struct page *page)
 		bad_reason = "non-NULL mapping";
 	if (unlikely(atomic_read(&page->_count) != 0))
 		bad_reason = "nonzero _count";
+	if (unlikely(page->flags & __PG_HWPOISON)) {
+		bad_reason = "HWPoisoned (hardware-corrupted)";
+		bad_flags = __PG_HWPOISON;
+	}
 	if (unlikely(page->flags & PAGE_FLAGS_CHECK_AT_PREP)) {
 		bad_reason = "PAGE_FLAGS_CHECK_AT_PREP flag set";
 		bad_flags = PAGE_FLAGS_CHECK_AT_PREP;
-- 
2.4.3

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