linux-kernel - [PATCH] mm: modify the method to search addr in unmapped

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Date:   Wed, 20 Apr 2022 16:40:39 +0800
From:   lipeifeng@...o.com
To:     akpm@...ux-foundation.org
Cc:     peifeng55@...il.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, 21cnbao@...il.com,
        zhangshiming@...o.com, lipeifeng <lipeifeng@...o.com>
Subject: [PATCH] mm: modify the method to search addr in unmapped_area

From: lipeifeng <lipeifeng@...o.com>

The old method will firstly find the space in len(info->length
+ info->align_mask), and get address at the desired alignment.

Sometime, addr  would be failed if there are enough
addr space in kernel by above method, e.g., you can't get a
addr sized in 1Mbytes, align_mask 1Mbytes successfully although
there are still (2M-1)bytes space in kernel.

This patch would fix thr problem above by the new method: find the
space in info->length and judge if at the desired info->align_mask
at the same time.

Do a simple test in TIF_32BIT with unmapped_area:
- Try to take addr (size:1M align:2M) until allocation fails;
- Try to take addr (size:1M align:1M) and account how to space can
be alloced successfully.

Before optimization: alloced 0     bytes.
After  optimization: alloced 1.9+G bytes.

Signed-off-by: lipeifeng <lipeifeng@...o.com>
---
 mm/mmap.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index a28ea5c..cb002f2 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1923,6 +1923,7 @@ static unsigned long unmapped_area(struct vm_unmapped_area_info *info)
 		return -ENOMEM;
 	low_limit = info->low_limit + length;
 
+	length = info->length;
 	/* Check if rbtree root looks promising */
 	if (RB_EMPTY_ROOT(&mm->mm_rb))
 		goto check_highest;
@@ -1944,6 +1945,8 @@ static unsigned long unmapped_area(struct vm_unmapped_area_info *info)
 		}
 
 		gap_start = vma->vm_prev ? vm_end_gap(vma->vm_prev) : 0;
+		/* Adjust gap address to the desired alignment */
+		gap_start += (info->align_offset - gap_start) & info->align_mask;
 check_current:
 		/* Check if current node has a suitable gap */
 		if (gap_start > high_limit)
@@ -1984,15 +1987,19 @@ static unsigned long unmapped_area(struct vm_unmapped_area_info *info)
 	gap_end = ULONG_MAX;  /* Only for VM_BUG_ON below */
 	if (gap_start > high_limit)
 		return -ENOMEM;
-
+	if (gap_start >= info->low_limit) {
+		gap_start += (info->align_offset - gap_start) & info->align_mask;
+		goto return_gap_start;
+	}
 found:
 	/* We found a suitable gap. Clip it with the original low_limit. */
-	if (gap_start < info->low_limit)
+	if (gap_start < info->low_limit) {
 		gap_start = info->low_limit;
+		/* Adjust gap address to the desired alignment */
+		gap_start += (info->align_offset - gap_start) & info->align_mask;
+	}
 
-	/* Adjust gap address to the desired alignment */
-	gap_start += (info->align_offset - gap_start) & info->align_mask;
-
+return_gap_start:
 	VM_BUG_ON(gap_start + info->length > info->high_limit);
 	VM_BUG_ON(gap_start + info->length > gap_end);
 	return gap_start;
-- 
2.7.4