linux-kernel - [PATCH] fs/cramfs: Convert kmap() to kmap_local

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Date:   Wed, 12 Oct 2022 11:13:56 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Nicolas Pitre <nico@...xnic.net>,
        Christian Brauner <brauner@...nel.org>,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        "Venkataramanan, Anirudh" <anirudh.venkataramanan@...el.com>,
        Ira Weiny <ira.weiny@...el.com>
Subject: [PATCH] fs/cramfs: Convert kmap() to kmap_local_data()

The use of kmap() is being deprecated in favor of kmap_local_page().

There are two main problems with kmap(): (1) It comes with an overhead as
the mapping space is restricted and protected by a global lock for
synchronization and (2) it also requires global TLB invalidation when the
kmap’s pool wraps and it might block when the mapping space is fully
utilized until a slot becomes available.

With kmap_local_page() the mappings are per thread, CPU local, can take
page faults, and can be called from any context (including interrupts).
It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore,
the tasks can be preempted and, when they are scheduled to run again, the
kernel virtual addresses are restored and still valid.

Since its use in fs/cramfs is safe everywhere, it should be preferred.

Therefore, replace kmap() with kmap_local_page() in fs/cramfs. Instead
of open-coding kmap_local_page() + memcpy(), use memcpy_from_page().

Cc: "Venkataramanan, Anirudh" <anirudh.venkataramanan@...el.com>
Suggested-by: Ira Weiny <ira.weiny@...el.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
---
 fs/cramfs/inode.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/fs/cramfs/inode.c b/fs/cramfs/inode.c
index 61ccf7722fc3..c17cbba5d45e 100644
--- a/fs/cramfs/inode.c
+++ b/fs/cramfs/inode.c
@@ -238,8 +238,7 @@ static void *cramfs_blkdev_read(struct super_block *sb, unsigned int offset,
 		struct page *page = pages[i];

 		if (page) {
-			memcpy(data, kmap(page), PAGE_SIZE);
-			kunmap(page);
+			memcpy_from_page(data, page, 0, PAGE_SIZE);
 			put_page(page);
 		} else
 			memset(data, 0, PAGE_SIZE);
@@ -815,7 +814,7 @@ static int cramfs_read_folio(struct file *file, struct folio *folio)

 	maxblock = (inode->i_size + PAGE_SIZE - 1) >> PAGE_SHIFT;
 	bytes_filled = 0;
-	pgdata = kmap(page);
+	pgdata = kmap_local_page(page);

 	if (page->index < maxblock) {
 		struct super_block *sb = inode->i_sb;
@@ -903,13 +902,13 @@ static int cramfs_read_folio(struct file *file, struct folio *folio)

 	memset(pgdata + bytes_filled, 0, PAGE_SIZE - bytes_filled);
 	flush_dcache_page(page);
-	kunmap(page);
+	kunmap_local(pgdata);
 	SetPageUptodate(page);
 	unlock_page(page);
 	return 0;

 err:
-	kunmap(page);
+	kunmap_local(pgdata);
 	ClearPageUptodate(page);
 	SetPageError(page);
 	unlock_page(page);
-- 
2.37.3