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    SubjectRe: [Kiobuf-io-devel] RFC: Kernel mechanism: Compound event wait /notify + callback chains
    Hi,

    On Sat, Feb 03, 2001 at 12:28:47PM -0800, Linus Torvalds wrote:
    >
    > On Thu, 1 Feb 2001, Stephen C. Tweedie wrote:
    > >
    > Neither the read nor the write are page-aligned. I don't know where you
    > got that idea. It's obviously not true even in the common case: it depends
    > _entirely_ on what the file offsets are, and expecting the offset to be
    > zero is just being stupid. It's often _not_ zero. With networking it is in
    > fact seldom zero, because the network packets are seldom aligned either in
    > size or in location.

    The underlying buffer is. The VFS (and the current kiobuf code) is
    already happy about IO happening at odd offsets within a page.
    However, the more general case --- doing zero-copy IO on arbitrary
    unaligned buffers --- simply won't work if you expect to be able to
    push those buffers to disk without a copy.

    The splice case you talked about is fine because it's doing the normal
    prepare/commit logic where the underlying buffer is page aligned, even
    if the splice IO is not to a page aligned location. That's _exactly_
    what kiobufs were intended to support. The prepare_read/prepare_write/
    pull/push cycle lets the caller tell the pull() function where to
    store its data, becausse there are alignment constraints which just
    can't be ignored: you simply cannot do physical disk IO on
    non-sector-aligned memory or in chunks which aren't a multiple of
    sector size. (The buffer address alignment can sometimes be relaxed
    --- obviously if you're doing PIO then it doesn't matter --- but the
    length granularity is rigidly enforced.)

    Cheers,
    Stephen
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