linux-kernel - Re: [PATCH] mm: Fix mmap_assert_locked() in follow

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Message-ID: <20240712124359.3199-1-spasswolf@web.de>
Date: Fri, 12 Jul 2024 14:43:57 +0200
From: Bert Karwatzki <spasswolf@....de>
To: Pei Li <peili.dev@...il.com>
Cc: Bert Karwatzki <spasswolf@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Shuah Khan <skhan@...uxfoundation.org>,
	David Hildenbrand <david@...hat.com>,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	linux-next@...r.kernel.org,
	syzbot+35a4414f6e247f515443@...kaller.appspotmail.com,
	syzkaller-bugs@...glegroups.com,
	linux-kernel-mentees@...ts.linuxfoundation.org,
	senozhatsky@...omium.org
Subject: Re: [PATCH] mm: Fix mmap_assert_locked() in follow_pte()

diff --git a/mm/memory.c b/mm/memory.c
index 282203363177..2f4b4322ec0e 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1817,6 +1817,7 @@ static void unmap_single_vma(struct mmu_gather *tlb,
 {
 	unsigned long start = max(vma->vm_start, start_addr);
 	unsigned long end;
+	bool mm_read_locked;

 	if (start >= vma->vm_end)
 		return;
@@ -1829,11 +1830,11 @@ static void unmap_single_vma(struct mmu_gather *tlb,

 	if (unlikely(vma->vm_flags & VM_PFNMAP)) {
 		if (!mm_wr_locked)
-			mmap_read_lock(vma->vm_mm);
+			mm_read_locked = !mmap_read_trylock(vma->vm_mm);

 		untrack_pfn(vma, 0, 0, mm_wr_locked);

-		if (!mm_wr_locked)
+		if (!mm_wr_locked && !mm_read_locked)
 			mmap_read_unlock(vma->vm_mm);
 	}


This seems to fix the issue without completely removing the locking.

Bert Karwatzki