Hi,

On Fri, Jan 20, 2017 at 9:41 PM, Suraj Choudhari
[off-list ref] wrote:

Hello,

I've some queries related to accessing the 'bio_vec' and 'bi_vcnt'
members in the BIO structure after the kernel changes implemented for
the 'immutable biovecs' -

Background -
-  From the changes done for the immutable biovecs, I understand that
the driver code now no longer needs to reference the 'bi_vcnt' and
'bi_io_vec' fields directly. Instead we can use the 'bvec_iter'
iterator in order to to access the bio_vec.
-  We've iterator functions bio_iter_iovec() and bio_advance_iter() as
well, which return us literal 'struct biovecs' taking into account the
bi_bvec_done and bi_size values.

Doubts  -
1)  Few functions in the block layer and some drivers still directly
refer to the 'bi_vcnt' and 'bio_vec' members of the BIO structure
[instead of accessing bio_vecs using the bvec_iter iterator].
Would there be some changes in the functions to compulsorily use the
'bvec_iter' in order to access bio_vecs?  If yes, are such changes

Generally speaking, yes, it is always better to use iterator helpers.

related to the 'multipage biovec' feature ?

Yes, it is a must for supporting multipage bvec.

2)  Can there be some problem if a driver directly accesses the
bi_io_vec [bio_vec] in the bio structure based on the values of bi_idx
and bi_vcnt ? [instead of using the bvec_iter iterator].

The direct access can't work any more once switching to multipage bvec,
and you can find some hint in the following link:

https://lkml.org/lkml/2017/1/15/266

Also .bi_vcnt can't be used for BIO_CLONED bio, and it is very easy
to make mistakes to access the table directly if someone don't understand
the iterator details enough.


Thanks,
Ming Lei