linux-kernel - Re: [PATCH resend] mm/page_alloc: fix comment is __get_free

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Date:   Thu, 30 Nov 2017 13:17:06 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     JianKang Chen <chenjiankang1@...wei.com>,
        mgorman@...hsingularity.net, hannes@...xchg.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        xieyisheng1@...wei.com, guohanjun@...wei.com,
        wangkefeng.wang@...wei.com
Subject: Re: [PATCH resend] mm/page_alloc: fix comment is __get_free_pages

On Thu, 30 Nov 2017 07:53:35 +0100 Michal Hocko <mhocko@...nel.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?