From: Johannes Thumshirn <jthumshirn@suse.de>
To: David Sterba <dsterba@suse.com>
Cc: Linux BTRFS Mailinglist <linux-btrfs@vger.kernel.org>,
Johannes Thumshirn <jthumshirn@suse.de>
Subject: [PATCH 3/3] btrfs: document extent mapping assumptions in checksum
Date: Tue, 27 Nov 2018 17:00:10 +0100 [thread overview]
Message-ID: <20181127160010.18123-4-jthumshirn@suse.de> (raw)
In-Reply-To: <20181127160010.18123-1-jthumshirn@suse.de>
Document why map_private_extent_buffer() cannot return '1' (i.e. the map
spans two pages) for the csum_tree_block() case.
The current algorithm for detecting a page boundary crossing in
map_private_extent_buffer() will return a '1' *IFF* the product of the
extent buffer's offset in the page + the offset passed in by
csum_tree_block() and the minimal length passed in by csum_tree_block() - 1
are bigger than PAGE_SIZE.
We always pass BTRFS_CSUM_SIZE (32) as offset and a minimal length of 32
and the current extent buffer allocator always guarantees page aligned
extends, so the above condition can't be true.
Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
---
fs/btrfs/disk-io.c | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 4bc270ef29b4..14d355d0cb7a 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -279,6 +279,12 @@ static int csum_tree_block(struct btrfs_fs_info *fs_info,
len = buf->len - offset;
while (len > 0) {
+ /*
+ * Note: we don't need to check for the err == 1 case here, as
+ * with the given combination of 'start = BTRFS_CSUM_SIZE (32)'
+ * and 'min_len = 32' and the currently implemented mapping
+ * algorithm we cannot cross a page boundary.
+ */
err = map_private_extent_buffer(buf, offset, 32,
&kaddr, &map_start, &map_len);
if (err)
--
2.16.4
next prev parent reply other threads:[~2018-11-27 16:00 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-27 16:00 [PATCH 0/3] Misc cosmetic changes for map_private_extent_buffer Johannes Thumshirn
2018-11-27 16:00 ` [PATCH 1/3] btrfs: don't initialize 'offset' in map_private_extent_buffer() Johannes Thumshirn
2018-11-27 16:31 ` Nikolay Borisov
2018-11-27 16:00 ` [PATCH 2/3] btrfs: use offset_in_page for start_offset " Johannes Thumshirn
2018-11-27 16:33 ` Nikolay Borisov
2018-11-27 16:00 ` Johannes Thumshirn [this message]
2018-11-27 16:36 ` [PATCH 3/3] btrfs: document extent mapping assumptions in checksum Nikolay Borisov
2018-11-27 19:08 ` Noah Massey
2018-11-27 19:32 ` Nikolay Borisov
2018-11-27 20:20 ` Noah Massey
2018-11-28 8:39 ` Johannes Thumshirn
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