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    SubjectRe: [dm-devel] [PATCH v5 12/12] block: Only clone bio vecs that are in use

    On Thu, Aug 9, 2012 at 12:01 AM, Tejun Heo <> wrote:
    > Hello,
    > On Wed, Aug 08, 2012 at 04:47:46PM -0700, Muthu Kumar wrote:
    >> You are changing the meaning of __bio_clone() here. In old code, the
    >> number of io_vecs, bi_idx, bi_vcnt are preserved. But in this modified
    >> code, you are mapping bio_src's bi_iovec[bi_idx] to bio_dests
    >> bi_iovec[0] and also restricting the number of allocated io_vecs of
    >> the clone. It may be useful for cases were we would like a identical
    >> copy of the original bio (may not be in current code base, but this
    >> implementation is definitely not what one would expect from the name
    >> "clone").
    > Implementation details changed somewhat but the high-level semantics
    > didn't change at all. Any driver not messing with bio internals - and
    > they shouldn't - shouldn't notice the change.

    The reason for doing this change is because the code in question is
    messing with bio internals.

    No in-kernel drivers
    > seem to be broken by the change. If you ask me, this looks more like
    > a bug fix to me where the bug is a silly behavior restricting
    > usefulness of the interface.
    >> May be, call this new implementation some thing else (and use it for bcache)?
    > This doesn't only change __bio_clone() but all clone interface stacked
    > on top of it, so, no way.

    >This ain't windows.

    ah... when you put it this way, it gets a different perspective :)

    Anyway, my point is, we shouldn't make it non-obvious ("clone" should
    be just "clone"). But, we can always add more comments i guess.


    > Thanks.
    > --
    > tejun

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