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    SubjectRe: O_DIRECT patch for processors with VIPT cache for mainline kernel (specifically arm in our case)
    On Thu, Nov 20, 2008 at 08:27:19AM -0700, Matthew Wilcox wrote:

    > I'm not quite sure why you need kmap_coherent(). If a page is mapped into
    > userspace, you can find what address it's mapped to from
    > page->mapping->i_mmap and page->index. OTOH, that's potentially

    Even if we know the userspace address of a page we do not necessarily have
    a usable mapping for kernel purposes. The userspace mapping might be r/o
    when we need r/w or it might be in another process. kmap_coherent takes
    the job of creating a r/w mapping on a suitable kernel virtual address
    that will avoid any aliases.

    > page->mapping->i_mmap and page->index. OTOH, that's potentially
    > expensive since you need to grab the spinlock, and unless you have all
    > user addresses coherent with each other (like parisc does), you need to
    > figure out which process to be coherent with.

    Having all userspace addresses of a page across all processes coherent with
    each other is the only practicable solution in Linux; at least I don't think
    how otherwise and within the currently kernel framework a platform could
    sanely handle userspace-userspace aliases. So we're talking about extending
    this to cover userspace-kernelspace aliases.

    The original reason for the introduction of kmap_coherent was avoiding
    a cache alias in when a multi-threaded process forks. The issue has been
    debated on lkml in 2006 as part of my submission of a patchset under the
    subject of "Fix COW D-cache aliasing on fork". The description is somewhat
    lengthy so I omit it here.

    One of the ugly parts of kmap_coherent() is that it cannot be used safely
    if the page has been marked as dirty by flush_dcache_page(); the callers
    know about this and deal with it.

    > I know James Bottomley did an experiment (and did an OLS presentation
    > ...) on unmapping the entire page cache and greatly expanding the kmap
    > area to do just this kind of thing. I think he even got a speedup.

    The speedup is no surprise.


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