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    SubjectRe: block device direct I/O fast path
    On Mon, Oct 31 2016, Christoph Hellwig wrote:
    > Hi Jens,
    >
    > this small series adds a fasth path to the block device direct I/O
    > code. It uses new magic created by Kent to avoid allocating an array
    > for the pages, and as part of that allows small, non-aio direct I/O
    > requests to be done without memory allocations or atomic ops and with
    > a minimal cache footprint. It's basically a cut down version of the
    > new iomap direct I/O code, and in the future it might also make sense
    > to move the main direct I/O code to a similar model. But indepedent
    > of that it's always worth to optimize the case of small, non-I/O
    > requests as allocating the bio and biovec on stack and a trivial
    > completion handler will always win over a full blown implementation.

    I'm not particularly tied to the request based implementation that I did
    here:

    http://git.kernel.dk/cgit/linux-block/log/?h=blk-dio

    I basically wanted to solve two problems with that:

    1) The slow old direct-io.c code
    2) Implement the hybrid polling

    #1 is accomplished with your code as well, though it does lack suppor
    for async IO, but that would not be hard to add.

    #2 is a bit more problematic, I'm pondering how we can implement that on
    top of the bio approach. The nice thing about the request based approach
    is that we have a 1:1 mapping with the unit on the driver side. And we
    have a place to store the timer. I don't particularly love the embedded
    timer, however, it'd be great to implement that differently. Trying to
    think of options there, haven't found any yet.

    --
    Jens Axboe

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