Re: [PATCH 3/3] btrfs: document extent mapping assumptions in checksum

[Nikolay Borisov <nborisov@suse.com>]

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)
>>>
>