linux-kernel - [PATCH] mm/page_poison: Replace kmap_atomic() with kmap_local

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Message-ID: <20231120142836.7219-1-fabio.maria.de.francesco@linux.intel.com>
Date:   Mon, 20 Nov 2023 15:28:23 +0100
From:   "Fabio M. De Francesco" <fabio.maria.de.francesco@...ux.intel.com>
To:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     "Fabio M. De Francesco" <fabio.maria.de.francesco@...ux.intel.com>,
        Ira Weiny <ira.weiny@...el.com>
Subject: [PATCH] mm/page_poison: Replace kmap_atomic() with kmap_local_page()

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

Therefore, replace kmap_atomic() with kmap_local_page().

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.

The code blocks between the mappings and un-mappings do not rely on the
above-mentioned side effects of kmap_atomic(), so that mere replacements
of the old API with the new one is all that they require (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/page_poison.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/page_poison.c b/mm/page_poison.c
index b4f456437b7e..3e9037363cf9 100644
--- a/mm/page_poison.c
+++ b/mm/page_poison.c
@@ -21,13 +21,13 @@ early_param("page_poison", early_page_poison_param);

 static void poison_page(struct page *page)
 {
-	void *addr = kmap_atomic(page);
+	void *addr = kmap_local_page(page);

 	/* KASAN still think the page is in-use, so skip it. */
 	kasan_disable_current();
 	memset(kasan_reset_tag(addr), PAGE_POISON, PAGE_SIZE);
 	kasan_enable_current();
-	kunmap_atomic(addr);
+	kunmap_local(addr);
 }

 void __kernel_poison_pages(struct page *page, int n)
@@ -77,7 +77,7 @@ static void unpoison_page(struct page *page)
 {
 	void *addr;

-	addr = kmap_atomic(page);
+	addr = kmap_local_page(page);
 	kasan_disable_current();
 	/*
 	 * Page poisoning when enabled poisons each and every page
@@ -86,7 +86,7 @@ static void unpoison_page(struct page *page)
 	 */
 	check_poison_mem(page, kasan_reset_tag(addr), PAGE_SIZE);
 	kasan_enable_current();
-	kunmap_atomic(addr);
+	kunmap_local(addr);
 }

 void __kernel_unpoison_pages(struct page *page, int n)
-- 
2.42.0