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

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Date:   Mon, 27 Nov 2023 16:55:21 +0100
From:   "Fabio M. De Francesco" <fabio.maria.de.francesco@...ux.intel.com>
To:     Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>,
        Vitaly Wool <vitaly.wool@...sulko.com>,
        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/zswap: 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() in
zswap.c.

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 mm/zswap.c, the blocks of code between the mappings and un-mappings
do not depend on the above-mentioned side effects of kmap_atomic(), so
that the 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/zswap.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 74411dfdad92..699c6ee11222 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1267,16 +1267,16 @@ bool zswap_store(struct folio *folio)
 	}

 	if (zswap_same_filled_pages_enabled) {
-		src = kmap_atomic(page);
+		src = kmap_local_page(page);
 		if (zswap_is_page_same_filled(src, &value)) {
-			kunmap_atomic(src);
+			kunmap_local(src);
 			entry->swpentry = swp_entry(type, offset);
 			entry->length = 0;
 			entry->value = value;
 			atomic_inc(&zswap_same_filled_pages);
 			goto insert_entry;
 		}
-		kunmap_atomic(src);
+		kunmap_local(src);
 	}

 	if (!zswap_non_same_filled_pages_enabled)
@@ -1422,9 +1422,9 @@ bool zswap_load(struct folio *folio)
 	spin_unlock(&tree->lock);

 	if (!entry->length) {
-		dst = kmap_atomic(page);
+		dst = kmap_local_page(page);
 		zswap_fill_page(dst, entry->value);
-		kunmap_atomic(dst);
+		kunmap_local(dst);
 		ret = true;
 		goto stats;
 	}
-- 
2.42.0