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    SubjectRe: [RFC][PATCH] flexible array implementation v3
    Dave Hansen wrote:
    > On Thu, 2009-07-23 at 10:44 +0900, KAMEZAWA Hiroyuki wrote:

    >> The caller can't know whether
    >> - there are no entry or
    >> - NULL(0) is stored at the array.
    >> Then, he has to check gotten value is valid or not by himself.
    > I think you may be reading this bit wrong. Either that, or I badly
    > miscoded it. :) The flex_array_get() code should returns NULL in cases
    > of error or the *address* of the data element. It will actually return
    > an address pointing into the second-level page.
    > NULL's two meaning here are:
    > 1. No element was ever put() here and thus there's no data page
    > 2. Your index was out of bounds
    > (1) is a little fuzzy here. It of course doesn't absolutely guarantee
    > that no put() was done since we just use the data page's existence to
    > tell. This is certainly no worse than a normal C array would be,
    > though.
    ok, maybe not necessary to handle.

    >> Can't we return -ENOENT here(especially when prealloc() is called) ?
    >> But ah, anyway, all-zero elements in allocated array exists ;(
    >> and the user can get value from an entry never be put.
    >> If we can fill the first 4bytes of _unused_ index by some magic code
    >> like this
    >> #define FLEX_ARRAY_UNUSED_MAGIC (0xa5a5a5a5)
    >> (if maintaining bitmap/status of usage is nonsense)
    >> and flex_array_get() can return -ENOENT, the users will feel easy.
    >> Overprotection ;) ?
    > I intentionally didn't use kzalloc() for the data pages. I figured that
    > users will either initialize it themselves or pass in __GFP_ZERO. I've
    > added a comment to clarify this.

    sorry for noise.
    I just felt that this code expects users know all they really do.
    As said, seems nice :) thank you.

    Reviewed-by: KAMEZAWA Hiroyuki <>

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