Re: [PATCH] ARM: mm: Regarding section when dealing with meminfo

From: Dave Hansen
Date: Mon Jan 24 2011 - 11:54:10 EST

On Sun, 2011-01-23 at 18:05 +0000, Russell King - ARM Linux wrote:
> On Thu, Jan 20, 2011 at 10:11:27AM -0800, Dave Hansen wrote:
> > On Thu, 2011-01-20 at 18:01 +0000, Russell King - ARM Linux wrote:
> > > > The x86 version of show_mem() actually manages to do this without any
> > > > #ifdefs, and works for a ton of configuration options. It uses
> > > > pfn_valid() to tell whether it can touch a given pfn.
> > >
> > > x86 memory layout tends to be very simple as it expects memory to
> > > start at the beginning of every region described by a pgdat and extend
> > > in one contiguous block. I wish ARM was that simple.
> >
> > x86 memory layouts can be pretty funky and have been that way for a long
> > time. That's why we *have* to handle holes in x86's show_mem(). My
> > laptop even has a ~1GB hole in its ZONE_DMA32:
> If x86 is soo funky, I suggest you try the x86 version of show_mem()
> on an ARM platform with memory holes. Make sure you try it with
> sparsemem as well...

x86 uses the generic lib/ show_mem(). It works for any holes, as long
as they're expressed in one of the memory models so that pfn_valid()
notices them.

ARM looks like its pfn_valid() is backed up by searching the (ASM
arch-specific) memblocks. That looks like it would be fairly slow
compared to the other pfn_valid() implementations and I can see why it's
being avoided in show_mem().

Maybe we should add either the MAX_ORDER or section_nr() trick to the
lib/ implementation. I bet that would use pfn_valid() rarely enough to
meet any performance concerns.

-- Dave

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