linux-kernel - [PATCH] MADVISE_FREE, THP: Fix madvise_free_huge

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Date:	Thu, 16 Jun 2016 20:03:54 -0700
From:	"Huang, Ying" <ying.huang@...el.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Huang Ying <ying.huang@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Jerome Marchand <jmarchan@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Ebru Akagunduz <ebru.akagunduz@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] MADVISE_FREE, THP: Fix madvise_free_huge_pmd return value after splitting

From: Huang Ying <ying.huang@...el.com>

madvise_free_huge_pmd should return 0 if the fallback PTE operations are
required.  In madvise_free_huge_pmd, if part pages of THP are discarded,
the THP will be split and fallback PTE operations should be used if
splitting succeeds.  But the original code will make fallback PTE
operations skipped, after splitting succeeds.  Fix that via make
madvise_free_huge_pmd return 0 after splitting successfully, so that the
fallback PTE operations will be done.

Know issues: if my understanding were correct, return 1 from
madvise_free_huge_pmd means the following processing for the PMD should
be skipped, while return 0 means the following processing is still
needed.  So the function should return 0 only if the THP is split
successfully or the PMD is not trans huge.  But the pmd_trans_unstable
after madvise_free_huge_pmd guarantee the following processing will be
skipped for huge PMD.  So current code can run properly.  But if my
understanding were correct, we can clean up return code of
madvise_free_huge_pmd accordingly.

Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
---
 mm/huge_memory.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 2ad52d5..64dc95d 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1655,14 +1655,9 @@ int madvise_free_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 	if (next - addr != HPAGE_PMD_SIZE) {
 		get_page(page);
 		spin_unlock(ptl);
-		if (split_huge_page(page)) {
-			put_page(page);
-			unlock_page(page);
-			goto out_unlocked;
-		}
+		split_huge_page(page);
 		put_page(page);
 		unlock_page(page);
-		ret = 1;
 		goto out_unlocked;
 	}

-- 
2.8.1