On Tue, Nov 27, 2018 at 11:43 AM Nikolay Borisov <[email protected]> wrote: > > On 27.11.18 г. 18:00 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 product of the > > I think the word product must be replaced with 'sum', since product > implies multiplication :) >
doesn't 'sum' imply addition? How about 'output'? > > 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 <[email protected]> > > With that wording changed: > > Reviewed-by: Nikolay Borisov <[email protected]> > > > --- > > 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) > >
