linux-kernel - [PATCH] btrfs: Replace kmap_atomic() with kmap_local

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Date:   Mon, 27 Jun 2022 19:48:49 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        Nick Terrell <terrelln@...com>,
        Chris Down <chris@...isdown.name>, Qu Wenruo <wqu@...e.com>,
        Filipe Manana <fdmanana@...e.com>,
        Gabriel Niebler <gniebler@...e.com>,
        linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Ira Weiny <ira.weiny@...el.com>, David Sterba <dsterba@...e.cz>
Subject: [PATCH] btrfs: Replace kmap_atomic() with kmap_local_page()

kmap_atomic() is being deprecated in favor of kmap_local_page() where it
is feasible. With kmap_local_page() mappings are per thread, CPU local,
and not globally visible.

As far as I can see, the kmap_atomic() calls in compression.c and in
inode.c can be safely converted.

Above all else, David Sterba has confirmed that "The context in
check_compressed_csum is atomic [...]" and that "kmap_atomic() in inode.c
[...] also can be replaced by kmap_local_page().".[1]

Therefore, convert all kmap_atomic() calls currently still left in fs/btrfs
to kmap_local_page().

Tested with xfstests on a QEMU + KVM 32-bits VM with 4GB RAM and booting a
kernel with HIGHMEM64GB enabled.

[1] https://lore.kernel.org/linux-btrfs/20220601132545.GM20
633@...n.jikos.cz/

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

Tests of groups "quick" and "compress" output several errors largely due
to memory leaks and shift-out-of-bounds. However, these errors are exactly
the same which are output without this and other conversions of mine to use
kmap_local_page(). Therefore, it looks like these changes don't introduce
regressions.

The previous RFC PATCH can be ignored:
https://lore.kernel.org/lkml/20220624084215.7287-1-fmdefrancesco@gmail.com/

With this patch, in fs/btrfs there are no longer call sites of kmap() and
kmap_atomic().

 fs/btrfs/compression.c |  4 ++--
 fs/btrfs/inode.c       | 12 ++++++------
 2 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
index f4564f32f6d9..b49719ae45b4 100644
--- a/fs/btrfs/compression.c
+++ b/fs/btrfs/compression.c
@@ -175,10 +175,10 @@ static int check_compressed_csum(struct btrfs_inode *inode, struct bio *bio,
 		/* Hash through the page sector by sector */
 		for (pg_offset = 0; pg_offset < bytes_left;
 		     pg_offset += sectorsize) {
-			kaddr = kmap_atomic(page);
+			kaddr = kmap_local_page(page);
 			crypto_shash_digest(shash, kaddr + pg_offset,
 					    sectorsize, csum);
-			kunmap_atomic(kaddr);
+			kunmap_local(kaddr);
 
 			if (memcmp(&csum, cb_sum, csum_size) != 0) {
 				btrfs_print_data_csum_error(inode, disk_start,
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index e921d6c432ac..0a7a621710f6 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -332,9 +332,9 @@ static int insert_inline_extent(struct btrfs_trans_handle *trans,
 			cur_size = min_t(unsigned long, compressed_size,
 				       PAGE_SIZE);
 
-			kaddr = kmap_atomic(cpage);
+			kaddr = kmap_local_page(cpage);
 			write_extent_buffer(leaf, kaddr, ptr, cur_size);
-			kunmap_atomic(kaddr);
+			kunmap_local(kaddr);
 
 			i++;
 			ptr += cur_size;
@@ -345,9 +345,9 @@ static int insert_inline_extent(struct btrfs_trans_handle *trans,
 	} else {
 		page = find_get_page(inode->vfs_inode.i_mapping, 0);
 		btrfs_set_file_extent_compression(leaf, ei, 0);
-		kaddr = kmap_atomic(page);
+		kaddr = kmap_local_page(page);
 		write_extent_buffer(leaf, kaddr, ptr, size);
-		kunmap_atomic(kaddr);
+		kunmap_local(kaddr);
 		put_page(page);
 	}
 	btrfs_mark_buffer_dirty(leaf);
@@ -3357,11 +3357,11 @@ static int check_data_csum(struct inode *inode, struct btrfs_bio *bbio,
 	offset_sectors = bio_offset >> fs_info->sectorsize_bits;
 	csum_expected = ((u8 *)bbio->csum) + offset_sectors * csum_size;
 
-	kaddr = kmap_atomic(page);
+	kaddr = kmap_local_page(page);
 	shash->tfm = fs_info->csum_shash;
 
 	crypto_shash_digest(shash, kaddr + pgoff, len, csum);
-	kunmap_atomic(kaddr);
+	kunmap_local(kaddr);
 
 	if (memcmp(csum, csum_expected, csum_size))
 		goto zeroit;
-- 
2.36.1