linux-kernel - [PATCH] mm/util: Use kmap_local_page() in memcmp

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Message-ID: <20231120141554.6612-1-fmdefrancesco@gmail.com>
Date:   Mon, 20 Nov 2023 15:15:27 +0100
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     Fabio De Francesco <fabio.maria.de.francesco@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        "Fabio M . De Francesco" <fabio.maria.de.francesco@...ux.intel.com>
Subject: [PATCH] mm/util: Use kmap_local_page() in memcmp_pages()

From: Fabio De Francesco <fabio.maria.de.francesco@...el.com>

kmap_atomic() has been deprecated in favor of kmap_local_page().

Therefore, replace kmap_atomic() with kmap_local_page() in
memcmp_pages().

kmap_atomic() is implemented like a kmap_local_page() which also
disables page-faults and preemption (the latter only in !PREEMPT_RT
kernels). The kernel virtual addresses returned by these two API are
only valid in the context of the callers (i.e., they cannot be handed to
other threads).

With kmap_local_page() the mappings are per thread and CPU local like
in kmap_atomic(); however, they can handle page-faults and can be called
from any context (including interrupts). The tasks that call
kmap_local_page() can be preempted and, when they are scheduled to run
again, the kernel virtual addresses are restored and are still valid.

In memcmp_pages(), the block of code between the mapping and un-mapping
does not depend on the above-mentioned side effects of kmap_aatomic(), so
that mere replacements of the old API with the new one is all that is
required (i.e., there is no need to explicitly call pagefault_disable()
and/or preempt_disable()).

Cc: Ira Weiny <ira.weiny@...el.com>
Signed-off-by: Fabio M. De Francesco <fabio.maria.de.francesco@...ux.intel.com>
---
 mm/util.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/util.c b/mm/util.c
index 744b4d7e3fae..5a6a9802583b 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -1047,11 +1047,11 @@ int __weak memcmp_pages(struct page *page1, struct page *page2)
 	char *addr1, *addr2;
 	int ret;

-	addr1 = kmap_atomic(page1);
-	addr2 = kmap_atomic(page2);
+	addr1 = kmap_local_page(page1);
+	addr2 = kmap_local_page(page2);
 	ret = memcmp(addr1, addr2, PAGE_SIZE);
-	kunmap_atomic(addr2);
-	kunmap_atomic(addr1);
+	kunmap_local(addr2);
+	kunmap_local(addr1);
 	return ret;
 }

-- 
2.42.0