linux-kernel - [PATCH v1 1/1] mm/ksm: fix inconsistent accounting of zero pages

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Date:   Thu,  5 Apr 2018 14:27:54 +0200
From:   Claudio Imbrenda <imbrenda@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     akpm@...ux-foundation.org, aarcange@...hat.com, minchan@...nel.org,
        kirill.shutemov@...ux.intel.com, linux-mm@...ck.org,
        hughd@...gle.com, borntraeger@...ibm.com,
        gerald.schaefer@...ibm.com, stable@...r.kernel.org
Subject: [PATCH v1 1/1] mm/ksm: fix inconsistent accounting of zero pages

When using KSM with use_zero_pages, we replace anonymous pages
containing only zeroes with actual zero pages, which are not anonymous.
We need to do proper accounting of the mm counters, otherwise we will
get wrong values in /proc and a BUG message in dmesg when tearing down
the mm.

Fixes: e86c59b1b1 ("mm/ksm: improve deduplication of zero pages with colouring")

Signed-off-by: Claudio Imbrenda <imbrenda@...ux.vnet.ibm.com>
---
 mm/ksm.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/ksm.c b/mm/ksm.c
index 293721f..2d6b352 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1131,6 +1131,13 @@ static int replace_page(struct vm_area_struct *vma, struct page *page,
 	} else {
 		newpte = pte_mkspecial(pfn_pte(page_to_pfn(kpage),
 					       vma->vm_page_prot));
+		/*
+		 * We're replacing an anonymous page with a zero page, which is
+		 * not anonymous. We need to do proper accounting otherwise we
+		 * will get wrong values in /proc, and a BUG message in dmesg
+		 * when tearing down the mm.
+		 */
+		dec_mm_counter(mm, MM_ANONPAGES);
 	}
 
 	flush_cache_page(vma, addr, pte_pfn(*ptep));
-- 
2.7.4