linux-kernel - [PATCH] mm: Always downgrade mmap

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Date:   Thu, 29 Jun 2023 20:14:14 +0100
From:   "Matthew Wilcox (Oracle)" <willy@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>
Cc:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Subject: [PATCH] mm: Always downgrade mmap_lock if requested

Now that stack growth must always hold the mmap_lock for write, we can
always downgrade the mmap_lock to read and safely unmap pages from the
page table, even if we're next to a stack.

Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
---
 mm/mmap.c | 15 ++-------------
 1 file changed, 2 insertions(+), 13 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 9b5188b65800..82efaca58ca2 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2550,19 +2550,8 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
 
 	mm->locked_vm -= locked_vm;
 	mm->map_count -= count;
-	/*
-	 * Do not downgrade mmap_lock if we are next to VM_GROWSDOWN or
-	 * VM_GROWSUP VMA. Such VMAs can change their size under
-	 * down_read(mmap_lock) and collide with the VMA we are about to unmap.
-	 */
-	if (downgrade) {
-		if (next && (next->vm_flags & VM_GROWSDOWN))
-			downgrade = false;
-		else if (prev && (prev->vm_flags & VM_GROWSUP))
-			downgrade = false;
-		else
-			mmap_write_downgrade(mm);
-	}
+	if (downgrade)
+		mmap_write_downgrade(mm);
 
 	/*
 	 * We can free page tables without write-locking mmap_lock because VMAs
-- 
2.39.2