| Message ID | 20181128085456.5702-4-jthumshirn@suse.de (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Misc cosmetic changes for map_private_extent_buffer | expand |
On 28.11.18 г. 10:54 ч., Johannes Thumshirn wrote: > 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 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> Reviewed-by: Nikolay Borisov <nborisov@suse.com> > > --- > Changes to v1: > * Changed wording of the commit message according to Noah's suggestion > --- > 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) >
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)
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 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> --- Changes to v1: * Changed wording of the commit message according to Noah's suggestion --- fs/btrfs/disk-io.c | 6 ++++++ 1 file changed, 6 insertions(+)