Thanks very much Ming and all !
I referred to multipage bvec patches link and it was indeed good & insightful.
I have few more query related to immutable bvecs & multipage bvecs, in
particular related to 4.4 kernel --
1) My understanding is bi_vcnt can't be used for a cloned BIO because
'bio_clone_fast' does NOT update the bi_vcnt field in the cloned BIO.
Is my understanding correct ?
2) For a block device driver, is it be safe to use the 'bi_vcnt' of a
BIO that's received from the upper layers, in order to access the
bvecs array, particularly in the '4.4' kernel ?
- I am thinking it may be safe to access bi_vcnt as long as the
block driver has NOT received a CLONE bio from upper layer. The
reason I think can be bvecs are not multipage in 4.4. Is my
understanding right ?
Thanks & Regards,
Suraj
On Fri, Jan 20, 2017 at 9:27 PM, Ming Lei <tom.leiming@xxxxxxxxx> wrote:
> Hi,
>
> On Fri, Jan 20, 2017 at 9:41 PM, Suraj Choudhari
> <surajschoudhari@xxxxxxxxx> 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
--
To unsubscribe from this list: send the line "unsubscribe linux-block" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
