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    SubjectRe: [PATCHSET pata-2.6] ide: rq->buffer, data, special and misc cleanups
    Hello, Boaz.

    Boaz Harrosh wrote:
    > I hope you have not been working on stabilizing this code yet, I've
    > just seen it this mornning (Israel time). It has obvious bugs, in
    > the use of scatterlists.

    Actually, I'm now and it's mostly working now.

    > But before you go running to fix them. Don't. I don't like the
    > scatter-list API at all. We where working hard to remove them from
    > scsi and I don't like them for block layer either.
    > If it is for my personal needs then all I need is a simple: "I have
    > this bio please attach it to a request". The reason I have a bio is
    > because it comes from a filesystem and because I will be needing to
    > use the raid engines that are bio based.
    > I must have confused you with my blk_rq_map_pages() patches, these
    > where made just as an example of the ugliness and fragility of not
    > using bios. It was suppose to be a bad example, not to be
    > accepted. (Hence the absence of my Singed-off-by:) Scatterlists are
    > bad because they are two times too fat for what they do here, they
    > need to be allocated, chained and slabbed, and finally they need to
    > be converted to a bio. Working directly with a bio is the shortest
    > route.

    sgl or bvec doesn't make much of difference. The only difference
    between sgl and bvec is the extra entries for DMA mapping. The
    problem is that there needs to be common way to describe the pages.
    The reason why the blk/bio mapping looks that complex is because all
    four cases - user map/copy and kern map/copy - are similar but subtly
    different. Without a common medium to carry the list of pages, each
    path ended up mixing what's common with what's different and thus the
    not quite manageable code.

    The reason why I want with sgl is because it's the sg mapping
    iterator. Other than that, using bvec or sgl wouldn't make any
    difference. All that's necessary is common carrier of page/offset/len

    But given the use of sgl there, I don't think the performance
    arguments holds much ground. How high performance or scalability is
    required in PC bio map paths? The added cost is at the most one more
    sgl allocation and use of more sparse data structure which BTW
    wouldn't be that large anyway. Both aren't worth worrying about
    considering how relative coldness of the path and other overhead of
    doing IO.

    If you think sgl is too heavy for things like this, the right way to
    go is to introduce some generic mechanism to carry sg chunks and use
    it in bio, but I don't think the overhead of the current sgl justifies
    that. If you have qualms about the sgl API, please clean it up. I
    just don't think bvec should be used outside of block/fs interface.
    As I wrote before, non-FS users have no reason to worry about bio.
    All it should think about is the requst it needs to issue and the
    memory area it's gonna use as in/out buffers. bio just doesn't have a
    place there.

    > I do not understand what was your intention for the
    > blk_rq_map_kern_sgl() how's the user for it?

    Well, the only in-kern user which was tinkering with rq/bio internals
    was scsi_req_map_sg(). Do you have another user?

    > BTW: now you absolutely must do something with the name of
    > blk_rq_map_sg() which does not do any mapping and does not change
    > request at all.

    Heh... yeap, definitely. Can you think of a good one?



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