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    SubjectRe: [PATCH resend] mm/page_alloc: fix comment is __get_free_pages
    On Thu, 30 Nov 2017 07:53:35 +0100 Michal Hocko <mhocko@kernel.org> wrote:

    > > mm... So we have a caller which hopes to be getting highmem pages but
    > > isn't. Caller then proceeds to pointlessly kmap the page and wonders
    > > why it isn't getting as much memory as it would like on 32-bit systems,
    > > etc.
    >
    > How he can kmap the page when he gets a _virtual_ address?

    doh.

    > > I do think we should help ferret out such bogosity. A WARN_ON_ONCE
    > > would suffice.
    >
    > This function has always been about lowmem pages. I seriously doubt we
    > have anybody confused and asking for a highmem page in the kernel. I
    > haven't checked that but it would already blow up as VM_BUG_ON tends to
    > be enabled on many setups.

    OK. But silently accepting __GFP_HIGHMEM is a bit weird - callers
    shouldn't be doing that in the first place.

    I wonder what happens if we just remove the WARN_ON and pass any
    __GFP_HIGHMEM straight through. The caller gets a weird address from
    page_to_virt(highmem page) and usually goes splat? Good enough
    treatment for something which never happens anyway?

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