linux-kernel - [PATCH v1] mm: drop NULL return check of pte_offset_map

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Date:   Thu, 18 May 2017 15:42:17 +0900
From:   Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:     linux-mm@...ck.org
Cc:     Mel Gorman <mgorman@...e.de>, Johannes Weiner <hannes@...xchg.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Subject: [PATCH v1] mm: drop NULL return check of pte_offset_map_lock()

pte_offset_map_lock() finds and takes ptl, and returns pte.
But some callers return without unlocking the ptl when pte == NULL,
which seems weird.

Git history said that !pte check in change_pte_range() was introduced in
commit 1ad9f620c3a2 ("mm: numa: recheck for transhuge pages under lock
during protection changes") and still remains after commit 175ad4f1e7a2
("mm: mprotect: use pmd_trans_unstable instead of taking the pmd_lock")
which partially reverts 1ad9f620c3a2. So I think that it's just dead code.

Many other caller of pte_offset_map_lock() never check NULL return, so
let's do likewise.

Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
---
 mm/memory.c   | 2 --
 mm/mprotect.c | 2 --
 2 files changed, 4 deletions(-)

diff --git v4.11-rc6-mmotm-2017-04-13-14-50/mm/memory.c v4.11-rc6-mmotm-2017-04-13-14-50_patched/mm/memory.c
index 8ae6700..c17fad1d 100644
--- v4.11-rc6-mmotm-2017-04-13-14-50/mm/memory.c
+++ v4.11-rc6-mmotm-2017-04-13-14-50_patched/mm/memory.c
@@ -4040,8 +4040,6 @@ static int __follow_pte_pmd(struct mm_struct *mm, unsigned long address,
 		goto out;
 
 	ptep = pte_offset_map_lock(mm, pmd, address, ptlp);
-	if (!ptep)
-		goto out;
 	if (!pte_present(*ptep))
 		goto unlock;
 	*ptepp = ptep;
diff --git v4.11-rc6-mmotm-2017-04-13-14-50/mm/mprotect.c v4.11-rc6-mmotm-2017-04-13-14-50_patched/mm/mprotect.c
index 8fd010f..d60a1ee 100644
--- v4.11-rc6-mmotm-2017-04-13-14-50/mm/mprotect.c
+++ v4.11-rc6-mmotm-2017-04-13-14-50_patched/mm/mprotect.c
@@ -58,8 +58,6 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
 	 * reading.
 	 */
 	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
-	if (!pte)
-		return 0;
 
 	/* Get target node for single threaded private VMAs */
 	if (prot_numa && !(vma->vm_flags & VM_SHARED) &&
-- 
2.7.0