On Tue, Nov 27, 2018 at 11:43 AM Nikolay Borisov <nborisov@xxxxxxxx> 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 <jthumshirn@xxxxxxx>
>
> With that wording changed:
>
> Reviewed-by: Nikolay Borisov <nborisov@xxxxxxxx>
>
> > ---
> > 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)
> >
