| Message ID | 20181127160010.18123-4-jthumshirn@suse.de (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Misc cosmetic changes for map_private_extent_buffer | expand |
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 :) > 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> With that wording changed: Reviewed-by: Nikolay Borisov <nborisov@suse.com> > --- > 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) >
On Tue, Nov 27, 2018 at 11:43 AM Nikolay Borisov <nborisov@suse.com> 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@suse.de> > > With that wording changed: > > Reviewed-by: Nikolay Borisov <nborisov@suse.com> > > > --- > > 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) > >
On 27.11.18 г. 21:08 ч., Noah Massey wrote: > On Tue, Nov 27, 2018 at 11:43 AM Nikolay Borisov <nborisov@suse.com> 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'? It does and the code indeed sums the value and not multiply them hence my suggestion. > >>> 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> >> >> With that wording changed: >> >> Reviewed-by: Nikolay Borisov <nborisov@suse.com> >> >>> --- >>> 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) >>> >
On Tue, Nov 27, 2018 at 2:32 PM Nikolay Borisov <nborisov@suse.com> wrote: > > On 27.11.18 г. 21:08 ч., Noah Massey wrote: > > On Tue, Nov 27, 2018 at 11:43 AM Nikolay Borisov <nborisov@suse.com> 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'? > > It does and the code indeed sums the value and not multiply them hence > my suggestion. > I'm sorry, I didn't phrase that well. Since 'sum' already implies addition, it gets confusing with the mathematical operators used later in the description. So, if a objective noun is required, a generic term such as 'output' or 'result' reads more cleanly for me. OTOH, dropping that and creating an actual expression *IFF* the extent buffer's offset in the page + the offset passed in by csum_tree_block() + the minimal length passed in by csum_tree_block() - 1 > PAGE_SIZE. is also straightforward.
On 27/11/2018 17:36, Nikolay Borisov 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 :) *doh* m) Yes thanks for spotting this.
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 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(+)