linux-kernel - Re: [PATCH v2] mm: page_mapped: don't assume compound page is huge or THP

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Date:   Mon, 3 Dec 2018 14:28:49 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Laszlo Ersek <lersek@...hat.com>
Cc:     Jan Stancek <jstancek@...hat.com>, linux-mm@...ck.org,
        alex.williamson@...hat.com, aarcange@...hat.com,
        rientjes@...gle.com, mgorman@...hsingularity.net, mhocko@...e.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: page_mapped: don't assume compound page is huge
 or THP

On Mon, Dec 03, 2018 at 11:23:58AM +0100, Laszlo Ersek wrote:
> Totally uninformed side-question:
> 
> how large can the return value of compound_order() be? MAX_ORDER?
> 
> Apparently, MAX_ORDER can be defined as CONFIG_FORCE_MAX_ZONEORDER.
> 
> "config FORCE_MAX_ZONEORDER" is listed in a number of Kconfig files.
> Among those, "arch/mips/Kconfig" permits "ranges" (?) that extend up to
> 64. Same applies to "arch/powerpc/Kconfig" and "arch/sh/mm/Kconfig".
> 
> If we left-shift "1" -- a signed int, which I assume in practice will
> always have two's complement representation, 1 sign bit, 31 value bits,
> and 0 padding bits --, by 31 or more bit positions, we get undefined
> behavior (as part of the left-shift operation).
> 
> Is this a practical concern?

Not really.

Assuming 4k PAGE_SIZE, compound_order() == 31 means 8 TiB pages. I doubt
we will see such allocation requests any time soon.

Even with 1k base page size, it's still 2 TiB.

We will see other limitations in page allocaiton path before the compund
order type will be an issue.

-- 
 Kirill A. Shutemov