Re: [RFC][patch 0/2] mm: remove PageReserved

[Russell King <rmk+lkml@arm.linux.org.uk>]

On Tue, Aug 09, 2005 at 07:29:30PM +1000, Nick Piggin wrote:
> Russell King wrote:
> > The usage of "valid ram" here is confusing - that's not what PageReserved
> > is all about.  It's about valid RAM which is managed by method other
> > than the usual page counting.  Non-reserved RAM is also valid RAM, but
> > is managed by the kernel in the usual way.
> 
> Well that is one usage of the PageReserved flag. That one tends
> to be easily covered by VM_RESERVED (ie. it is no longer used that
> way after the patches).
> 
> The remaining problem is, in fact, these "other" uses of PageReserved.
> One usage definitely appears to be "is this page valid RAM?".

Hmm, that sounds like an architecture specific extension above the
basic requirements.

> > The former is available for remap_pfn_range and ioremap, the latter is
> > not.
> 
> I thought ioremap was attempting to avoid remapping physical
> RAM with that check. All drivers I have looked at which allocate
> physical memory then SetPageReserved the pages use remap_pfn_range
> but I admit that's not a huge number (that I have looked at).

They do this because:

1. they want to control when this RAM is freed.
2. remap_pfn_range refuses to map RAM that isn't marked reserved.

To put it another way, they fiddle with the reserved bit because
that's what the current interfaces forces upon them.  I would
dearly like that to go away though.

> > On the other hand, the validity of an apparant RAM address can only be
> > tested using its pfn with pfn_valid().
> 
> I'm fairly sure that's not the case on i386 at least. I think
> pfn_valid will be true if the pfn points to a struct page.
> See arch/i386/mm/init.c:one_highpage_init()

This sounds like i386 is doing something which is a superset of the
base requirements, which is an architecture specific extension.  No
problem with that, but that's i386 folk's problem. 8)

Ok, but I still disagree with replacing something called reserved
with something which leads one to believe that it's intended for
checking whether a struct page is "valid" RAM or not when there's
other interfaces which are supposed to be used for that.

I wonder if we can optimise out the useless "valid" RAM checks
on architectures which don't require this insanity...

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 Serial core
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