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

[Kirill A. Shutemov]

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