linux-kernel - [PATCH] mm/ksm: Use kmap_local_page() in calc

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Date:   Mon, 20 Nov 2023 15:18:44 +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 M. 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/ksm: Use kmap_local_page() in calc_checksum()

From: "Fabio M. 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
calc_checksum().

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 calc_checksum(), the block of code between the mapping and un-mapping
does not depend on the above-mentioned side effects of kmap_aatomic(), so
that a 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/ksm.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 6a831009b4cb..5d60d5385de6 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1099,9 +1099,9 @@ static int unmerge_and_remove_all_rmap_items(void)
 static u32 calc_checksum(struct page *page)
 {
 	u32 checksum;
-	void *addr = kmap_atomic(page);
+	void *addr = kmap_local_page(page);
 	checksum = xxhash(addr, PAGE_SIZE, 0);
-	kunmap_atomic(addr);
+	kunmap_local(addr);
 	return checksum;
 }

-- 
2.42.0