On Thu, Feb 2, 2017 at 5:35 PM, Suraj Choudhari <surajschoudhari@xxxxxxxxx> wrote: > Thanks very much Ming and all ! You are welcome! > > 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 ? Yes and no. A cloned BIO may just map to a range of vectors from the source bio via helpers. > > 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 ? No, as we mentioned, it isn't safe to do that, especially after v4.3 introduces bio splitting, which may cause a block driver to see a cloned bio easily. Also dm may pass a cloned a bio to block driver too. So please use bio helpers to iterate the bvec instead of direct access. > - 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 ? Yes, but why not use use the helpers transparently for both cloned bio and non-cloned? Thanks, Ming > > 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 -- Ming Lei
