linux-kernel - [PATCH v2] mm/hugetlb: Fix calculation of adjust_range_if_pmd_sharing

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Date:   Thu, 30 Jul 2020 16:16:36 -0400
From:   Peter Xu <peterx@...hat.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     peterx@...hat.com, Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: [PATCH v2] mm/hugetlb: Fix calculation of adjust_range_if_pmd_sharing_possible

This is found by code observation only.

Firstly, the worst case scenario should assume the whole range was covered by
pmd sharing.  The old algorithm might not work as expected for ranges
like (1g-2m, 1g+2m), where the adjusted range should be (0, 1g+2m) but the
expected range should be (0, 2g).

Since at it, remove the loop since it should not be required.  With that, the
new code should be faster too when the invalidating range is huge.

CC: Andrea Arcangeli <aarcange@...hat.com>
CC: Mike Kravetz <mike.kravetz@...cle.com>
CC: Andrew Morton <akpm@...ux-foundation.org>
CC: Matthew Wilcox <willy@...radead.org>
CC: linux-mm@...ck.org
CC: linux-kernel@...r.kernel.org
Signed-off-by: Peter Xu <peterx@...hat.com>
---
v2:
- use min/max instead of custom MIN/MAX [Matthew]
---
 mm/hugetlb.c | 24 ++++++++++--------------
 1 file changed, 10 insertions(+), 14 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 4645f1441d32..7332f3c4b8ec 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5321,25 +5321,21 @@ static bool vma_shareable(struct vm_area_struct *vma, unsigned long addr)
 void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
 				unsigned long *start, unsigned long *end)
 {
-	unsigned long check_addr;
+	unsigned long a_start, a_end;
 
 	if (!(vma->vm_flags & VM_MAYSHARE))
 		return;
 
-	for (check_addr = *start; check_addr < *end; check_addr += PUD_SIZE) {
-		unsigned long a_start = check_addr & PUD_MASK;
-		unsigned long a_end = a_start + PUD_SIZE;
+	/* Extend the range to be PUD aligned for a worst case scenario */
+	a_start = ALIGN_DOWN(*start, PUD_SIZE);
+	a_end = ALIGN(*end, PUD_SIZE);
 
-		/*
-		 * If sharing is possible, adjust start/end if necessary.
-		 */
-		if (range_in_vma(vma, a_start, a_end)) {
-			if (a_start < *start)
-				*start = a_start;
-			if (a_end > *end)
-				*end = a_end;
-		}
-	}
+	/*
+	 * Intersect the range with the vma range, since pmd sharing won't be
+	 * across vma after all
+	 */
+	*start = max(vma->vm_start, a_start);
+	*end = min(vma->vm_end, a_end);
 }
 
 /*
-- 
2.26.2