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    SubjectRe: [PATCH v2] mmap.2: MAP_FIXED updated documentation
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    On 12/04/2017 02:55 AM, Cyril Hrubis wrote:
    > Hi!
    > I know that we are not touching the rest of the existing description for
    > MAP_FIXED however the second sentence in the manual page says that "addr
    > must be a multiple of the page size." Which however is misleading as
    > this is not enough on some architectures. Code in the wild seems to
    > (mis)use SHMLBA for aligment purposes but I'm not sure that we should
    > advise something like that in the manpages.
    >
    > So what about something as:
    >
    > "addr must be suitably aligned, for most architectures multiple of page
    > size is sufficient, however some may impose additional restrictions for
    > page mapping addresses."
    >

    Hi Cyril,

    Right, so I've been looking into this today, and I think we can go a bit
    further than that, even. The kernel, as far back as the *original* git
    commit in 2005, implements mmap on ARM by requiring that the address is
    aligned to SHMLBA:

    arch/arm/mm/mmap.c:50:

    if (flags & MAP_FIXED) {
    if (aliasing && flags & MAP_SHARED &&
    (addr - (pgoff << PAGE_SHIFT)) & (SHMLBA - 1))
    return -EINVAL;
    return addr;
    }

    So, given that this has been the implementation for the last 12+ years (and
    probably the whole time, in fact), I think we can be bold enough to use this
    wording for the second sentence of MAP_FIXED:

    "addr must be a multiple of SHMLBA (<sys/shm.h>), which in turn is either
    the system page size (on many architectures) or a multiple of the system
    page size (on some architectures)."

    What do you think?

    thanks,
    John Hubbard
    NVIDIA

    > Which should at least hint the reader that this is architecture specific.
    >

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