linux-ext4 - [PATCH] fs/ext2: Replace kmap_atomic() with kmap_local

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Date:   Sat, 31 Dec 2022 18:42:05 +0100
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Jan Kara <jack@...e.com>, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Ira Weiny <ira.weiny@...el.com>
Subject: [PATCH] fs/ext2: Replace kmap_atomic() with kmap_local_page()

kmap_atomic() is 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 for !PREEMPT_RT kernels).

However, the code within the mapping and un-mapping in ext2_make_empty()
does not depend on the above-mentioned side effects.

Therefore, a mere replacement of the old API with the new one is all it
is required (i.e., there is no need to explicitly add any calls to
pagefault_disable() and/or preempt_disable()).

Suggested-by: Ira Weiny <ira.weiny@...el.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
---

I tried my best to understand the code within mapping and un-mapping.
However, I'm not an expert. Therefore, although I'm pretty confident, I
cannot be 100% sure that the code between the mapping and the un-mapping
does not depend on pagefault_disable() and/or preempt_disable().

Unfortunately, I cannot currently test this changes to check the
above-mentioned assumptions. However, if I'm required to do the tests
with (x)fstests, I have no problems with doing them in the next days.

If so, I'll test in a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel with
HIGHMEM64GB enabled.

I'd like to hear whether or not the maintainers require these tests
and/or other tests.

 fs/ext2/dir.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/ext2/dir.c b/fs/ext2/dir.c
index e5cbc27ba459..0f144c5c7861 100644
--- a/fs/ext2/dir.c
+++ b/fs/ext2/dir.c
@@ -646,7 +646,7 @@ int ext2_make_empty(struct inode *inode, struct inode *parent)
 		unlock_page(page);
 		goto fail;
 	}
-	kaddr = kmap_atomic(page);
+	kaddr = kmap_local_page(page);
 	memset(kaddr, 0, chunk_size);
 	de = (struct ext2_dir_entry_2 *)kaddr;
 	de->name_len = 1;
@@ -661,7 +661,7 @@ int ext2_make_empty(struct inode *inode, struct inode *parent)
 	de->inode = cpu_to_le32(parent->i_ino);
 	memcpy (de->name, "..\0", 4);
 	ext2_set_de_type (de, inode);
-	kunmap_atomic(kaddr);
+	kunmap_local(kaddr);
 	ext2_commit_chunk(page, 0, chunk_size);
 	err = ext2_handle_dirsync(inode);
 fail:
-- 
2.39.0