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

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Date:   Mon, 20 Nov 2023 15:26:31 +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/mempool: 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 don't 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/mempool.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/mempool.c b/mm/mempool.c
index 734bcf5afbb7..b3d2084fd989 100644
--- a/mm/mempool.c
+++ b/mm/mempool.c
@@ -64,10 +64,10 @@ static void check_element(mempool_t *pool, void *element)
 	} else if (pool->free == mempool_free_pages) {
 		/* Mempools backed by page allocator */
 		int order = (int)(long)pool->pool_data;
-		void *addr = kmap_atomic((struct page *)element);
+		void *addr = kmap_local_page((struct page *)element);

 		__check_element(pool, addr, 1UL << (PAGE_SHIFT + order));
-		kunmap_atomic(addr);
+		kunmap_local(addr);
 	}
 }

@@ -89,10 +89,10 @@ static void poison_element(mempool_t *pool, void *element)
 	} else if (pool->alloc == mempool_alloc_pages) {
 		/* Mempools backed by page allocator */
 		int order = (int)(long)pool->pool_data;
-		void *addr = kmap_atomic((struct page *)element);
+		void *addr = kmap_local_page((struct page *)element);

 		__poison_element(addr, 1UL << (PAGE_SHIFT + order));
-		kunmap_atomic(addr);
+		kunmap_local(addr);
 	}
 }
 #else /* CONFIG_DEBUG_SLAB || CONFIG_SLUB_DEBUG_ON */
-- 
2.42.0