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    SubjectRe: mmap: is default non-populating behavior stable?
    On Tue, 4 Nov 2008, Eugene V. Lyubimkin wrote:
    > Alan Cox wrote:
    > >
    > > I believe our behaviour is correct for mmap/mumap/truncate and it
    > > certainly used to be and was tested.


    > >
    > > At the point you do anything involving mremap (which is non posix) our
    > > behaviour becomes rather bizarre.

    Certainly mremap is non-POSIX, but I can't think of any way in which
    it would interfere with Eugene's assumptions about population.

    (Every year or so we do wonder whether to change an extending mremap
    of a MAP_SHARED|MAP_ANONYMOUS object to extend the object itself instead
    of just SIGBUSing on the extension: but I've so far remained conservative
    about that, and Eugene appears to be thinking of more ordinary files.)

    > Thanks to all for answers. I have made the conclusion that doing "open() new
    > file, truncate(<big size>), mmap(<the same big size>), write/read some memory
    > pages" should not populate other, untouched by write/read pages (until
    > MAP_POPULATE given), right?

    That is a reasonable description of how the kernel tries and will always
    try to handle it, approximately; but I don't think you can rely upon it

    For a start, it depends on the filesystem: I believe that vfat, for
    example, does not support the concept of sparse files (files with holes
    in), so its truncate(<big size>) will allocate the whole of that big
    size initially.

    I'm not sure what you mean by "populate": in mm, as in MAP_POPULATE,
    we're thinking of prefaulting pages into the user address space; but
    you're probably thinking of whether the blocks are allocated on disk?

    Prefaulting hole pages into the user address space may imply allocating
    blocks on disk, or it may not: likely to depend on filesystem again.

    From time to time we toy with prefaulting adjacent pages when a fault
    occurs (though IIRC tests have proved disappointing in the past): we'd
    like to keep that option open, but it would go against your guidelines
    above to some extent.


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