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From:   Rolf Eike Beer <eb@emlix.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: [PATCH 5/6] mm: pagewalk: allow walk_page_range_novma() without mm
Date:   Mon, 22 Aug 2022 15:03:29 +0200
Message-ID: <5760214.MhkbZ0Pkbq@devpool047>
Organization: emlix GmbH
In-Reply-To: <3200642.44csPzL39Z@devpool047>
References: <3200642.44csPzL39Z@devpool047>
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Since e47690d756a7 ("x86: mm: avoid allocating struct mm_struct on the
stack") a pgd can be passed to walk_page_range_novma(). In case it is set
no place in the pagewalk code use the walk.mm anymore, so permit to pass a
NULL mm instead. It is up to the caller to ensure proper locking on the pgd
in this case.

Signed-off-by: Rolf Eike Beer <eb@emlix.com>
---
 mm/pagewalk.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index dc3c850cd148..f816f86588be 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -506,6 +506,8 @@ int walk_page_range(struct mm_struct *mm, unsigned long start,
  * not backed by VMAs. Because 'unusual' entries may be walked this function
  * will also not lock the PTEs for the pte_entry() callback. This is useful for
  * walking the kernel pages tables or page tables for firmware.
+ *
+ * Either mm or pgd may be NULL, but not both.
  */
 int walk_page_range_novma(struct mm_struct *mm, unsigned long start,
 			  unsigned long end, const struct mm_walk_ops *ops,
@@ -520,10 +522,11 @@ int walk_page_range_novma(struct mm_struct *mm, unsigned long start,
 		.no_vma		= true
 	};
 
-	if (start >= end || !walk.mm)
+	if (start >= end || (!walk.mm && !walk.pgd))
 		return -EINVAL;
 
-	mmap_assert_locked(walk.mm);
+	if (walk.mm)
+		mmap_assert_locked(walk.mm);
 
 	return walk_pgd_range(start, end, &walk);
 }
-- 
2.37.2

-- 
Rolf Eike Beer, emlix GmbH, http://www.emlix.com
Fon +49 551 30664-0, Fax +49 551 30664-11
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