Re: [PATCH for-next v7 4/5] block: add helper to map bvec iterator for passthrough

[Kanchan Joshi <joshi.k@samsung.com>]

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On Mon, Sep 26, 2022 at 04:50:40PM +0200, Christoph Hellwig wrote:
>On Sat, Sep 24, 2022 at 12:13:49AM +0530, Kanchan Joshi wrote:
>> And efficency is the concern as we are moving to more heavyweight
>> helper that 'handles' weird conditions rather than just 'bails out'.
>> These alignment checks end up adding a loop that traverses
>> the entire ITER_BVEC.
>> Also blk_rq_map_user_iov uses bio_iter_advance which also seems
>> cycle-consuming given below code-comment in io_import_fixed():
>
>No one says you should use the existing loop in blk_rq_map_user_iov.
>Just make it call your new helper early on when a ITER_BVEC iter is
>passed in.

Indeed. I will send the v10 with that approach.

>> Do you see good way to trigger this virt-alignment condition? I have
>> not seen this hitting (the SG gap checks) when running with fixebufs.
>
>You'd need to make sure the iovec passed to the fixed buffer
>registration is chunked up smaller than the nvme page size.
>
>E.g. if you pass lots of non-contiguous 512 byte sized iovecs to the
>buffer registration.
>
>>> We just need to implement the equivalent functionality for bvecs.  It
>>> isn't really hard, it just wasn't required so far.
>>
>> Can the virt-boundary alignment gap exist for ITER_BVEC iter in first
>> place?
>
>Yes.  bvecs are just a way to represent data.  If the individual
>segments don't fit the virt boundary you still need to deal with it.

Thanks for clearing this.

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