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    SubjectRe: mmap/munmap semantics

    On Tue, 22 Feb 2000 18:46:02 +0100 (MET), Richard Guenther
    <> said:

    > With the ongoing development of GLAME there arise the following
    > problems with the backing-store management, which is a mmaped
    > file and does "userspace virtual memory management":
    > - I cannot see a way to mmap a part of the file but set the
    > contents initially to zero,

    All file contents default to zero anyway, so just ftruncate() the file
    to create as much demand-zeroed mmapable memory as you want.

    > - I need to "drop" a mapping sometimes without writing the contents
    > back to disk - I cannot see a way to do this with linux currently.

    The only way is to use Chuck Lever's madvise() patches:
    madvise(MADV_DONTNEED) is exactly what you need there. It's not yet in
    Linus's 2.3 tree, but the API is pretty standard.

    > Ideally a hole could be created in the mmapped file on drop time -

    No, if the mmaped area has already been flushed to disk then there is no
    way at all to recreate the hole except by truncating and then
    re-extending the file (which destroys everything until EOF, of course).


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