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    SubjectRe: [fuse-devel] [PATCH] FUSE/CUSE: implement direct mmap support
    Hello,

    On 02/11/2010 10:40 PM, Miklos Szeredi wrote:
    > On Thu, 11 Feb 2010, Tejun Heo wrote:
    >>> So there are different offsets:
    >>>
    >>> a) vma->vm_pgoff (which may mean anything, but usually means b)
    >>
    >> Yeap, vma->vm_pgoff can be any value and doesn't really matter. The
    >> only visible difference would be the /proc listing, right? Setting
    >> this to the requested offset is trivial.
    >
    > You mean leaving it at the requested offset? Yes, that's the most
    > trivial thing to do. Very few drivers change vm_pgoff:
    >
    > git grep "vm_pgoff *=[^=]"

    Yeap, sure. I just didn't think it was visible outside.

    >>> b) the offset at which the pages of the mapping are located
    >>> c) the offset at which the server side mmap is located
    >>
    >> There are three offsets.
    >>
    >> a) the offset a client requested
    >>
    >> b) the offset into dmmap AS, a client mmap region is mapped to. This
    >> could be different from a) by multiple of SHMLBA / PAGE_SIZE.
    >
    > No, it could be different from a) by an arbitrary value.

    Then, sharing those pages would cause aliasing issues.

    >> c) the offset into dmmap AS, a server mmap region is mapped to, where
    >> collection of these mmaps define the dmmap AS.
    >>
    >> The offsets used in b) and c) are the same offsets.
    >
    > Why are they the same?

    I meant they point into the same space. If they're the same value,
    they point to the same page.

    Thanks.

    --
    tejun


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