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

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Message-Id: <20221231174439.8557-1-fmdefrancesco@gmail.com>
Date:   Sat, 31 Dec 2022 18:44:39 +0100
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     "Theodore Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        linux-fsdevel@...r.kernel.org
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Ira Weiny <ira.weiny@...el.com>
Subject: [PATCH] fs/ext4: 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 mappings and un-mappings in ext4/inline.c
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/ext4/inline.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/fs/ext4/inline.c b/fs/ext4/inline.c
index 2b42ececa46d..bfb044425d8a 100644
--- a/fs/ext4/inline.c
+++ b/fs/ext4/inline.c
@@ -490,10 +490,10 @@ static int ext4_read_inline_page(struct inode *inode, struct page *page)
 		goto out;

 	len = min_t(size_t, ext4_get_inline_size(inode), i_size_read(inode));
-	kaddr = kmap_atomic(page);
+	kaddr = kmap_local_page(page);
 	ret = ext4_read_inline_data(inode, kaddr, len, &iloc);
 	flush_dcache_page(page);
-	kunmap_atomic(kaddr);
+	kunmap_local(kaddr);
 	zero_user_segment(page, len, PAGE_SIZE);
 	SetPageUptodate(page);
 	brelse(iloc.bh);
@@ -763,9 +763,9 @@ int ext4_write_inline_data_end(struct inode *inode, loff_t pos, unsigned len,
 		 */
 		(void) ext4_find_inline_data_nolock(inode);

-		kaddr = kmap_atomic(page);
+		kaddr = kmap_local_page(page);
 		ext4_write_inline_data(inode, &iloc, kaddr, pos, copied);
-		kunmap_atomic(kaddr);
+		kunmap_local(kaddr);
 		SetPageUptodate(page);
 		/* clear page dirty so that writepages wouldn't work for us. */
 		ClearPageDirty(page);
@@ -831,9 +831,9 @@ ext4_journalled_write_inline_data(struct inode *inode,
 	}

 	ext4_write_lock_xattr(inode, &no_expand);
-	kaddr = kmap_atomic(page);
+	kaddr = kmap_local_page(page);
 	ext4_write_inline_data(inode, &iloc, kaddr, 0, len);
-	kunmap_atomic(kaddr);
+	kunmap_local(kaddr);
 	ext4_write_unlock_xattr(inode, &no_expand);

 	return iloc.bh;
-- 
2.39.0