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

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Message-ID: <20231127155452.586387-1-fabio.maria.de.francesco@linux.intel.com>
Date:   Mon, 27 Nov 2023 16:54:37 +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/swapfile: 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
swapfile.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/swapfile.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/swapfile.c | 34 +++++++++++++++++-----------------
 1 file changed, 17 insertions(+), 17 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 4bc70f459164..8be70912e298 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1495,9 +1495,9 @@ int swp_swapcount(swp_entry_t entry)
 
 	do {
 		page = list_next_entry(page, lru);
-		map = kmap_atomic(page);
+		map = kmap_local_page(page);
 		tmp_count = map[offset];
-		kunmap_atomic(map);
+		kunmap_local(map);
 
 		count += (tmp_count & ~COUNT_CONTINUED) * n;
 		n *= (SWAP_CONT_MAX + 1);
@@ -3477,9 +3477,9 @@ int add_swap_count_continuation(swp_entry_t entry, gfp_t gfp_mask)
 		if (!(count & COUNT_CONTINUED))
 			goto out_unlock_cont;
 
-		map = kmap_atomic(list_page) + offset;
+		map = kmap_local_page(list_page) + offset;
 		count = *map;
-		kunmap_atomic(map);
+		kunmap_local(map);
 
 		/*
 		 * If this continuation count now has some space in it,
@@ -3529,7 +3529,7 @@ static bool swap_count_continued(struct swap_info_struct *si,
 	spin_lock(&si->cont_lock);
 	offset &= ~PAGE_MASK;
 	page = list_next_entry(head, lru);
-	map = kmap_atomic(page) + offset;
+	map = kmap_local_page(page) + offset;
 
 	if (count == SWAP_MAP_MAX)	/* initial increment from swap_map */
 		goto init_map;		/* jump over SWAP_CONT_MAX checks */
@@ -3539,27 +3539,27 @@ static bool swap_count_continued(struct swap_info_struct *si,
 		 * Think of how you add 1 to 999
 		 */
 		while (*map == (SWAP_CONT_MAX | COUNT_CONTINUED)) {
-			kunmap_atomic(map);
+			kunmap_local(map);
 			page = list_next_entry(page, lru);
 			BUG_ON(page == head);
-			map = kmap_atomic(page) + offset;
+			map = kmap_local_page(page) + offset;
 		}
 		if (*map == SWAP_CONT_MAX) {
-			kunmap_atomic(map);
+			kunmap_local(map);
 			page = list_next_entry(page, lru);
 			if (page == head) {
 				ret = false;	/* add count continuation */
 				goto out;
 			}
-			map = kmap_atomic(page) + offset;
+			map = kmap_local_page(page) + offset;
 init_map:		*map = 0;		/* we didn't zero the page */
 		}
 		*map += 1;
-		kunmap_atomic(map);
+		kunmap_local(map);
 		while ((page = list_prev_entry(page, lru)) != head) {
-			map = kmap_atomic(page) + offset;
+			map = kmap_local_page(page) + offset;
 			*map = COUNT_CONTINUED;
-			kunmap_atomic(map);
+			kunmap_local(map);
 		}
 		ret = true;			/* incremented */
 
@@ -3569,21 +3569,21 @@ init_map:		*map = 0;		/* we didn't zero the page */
 		 */
 		BUG_ON(count != COUNT_CONTINUED);
 		while (*map == COUNT_CONTINUED) {
-			kunmap_atomic(map);
+			kunmap_local(map);
 			page = list_next_entry(page, lru);
 			BUG_ON(page == head);
-			map = kmap_atomic(page) + offset;
+			map = kmap_local_page(page) + offset;
 		}
 		BUG_ON(*map == 0);
 		*map -= 1;
 		if (*map == 0)
 			count = 0;
-		kunmap_atomic(map);
+		kunmap_local(map);
 		while ((page = list_prev_entry(page, lru)) != head) {
-			map = kmap_atomic(page) + offset;
+			map = kmap_local_page(page) + offset;
 			*map = SWAP_CONT_MAX | count;
 			count = COUNT_CONTINUED;
-			kunmap_atomic(map);
+			kunmap_local(map);
 		}
 		ret = count == COUNT_CONTINUED;
 	}
-- 
2.42.0