Re: [RFC] Tight check of pfn_valid on sparsemem

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Wed, 14 Jul 2010 16:35:22 +0900
Minchan Kim <minchan.kim@gmail.com> wrote:

> On Wed, Jul 14, 2010 at 4:10 PM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > On Wed, 14 Jul 2010 15:44:41 +0900
> > Minchan Kim <minchan.kim@gmail.com> wrote:
> >
> >> Hi, Kame.
> >>
> >> On Wed, Jul 14, 2010 at 9:23 AM, KAMEZAWA Hiroyuki
> >> <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> >> > On Wed, 14 Jul 2010 01:44:23 +0900
> >> > Minchan Kim <minchan.kim@gmail.com> wrote:
> >> >
> >> >> > If you _really_ can't make the section size smaller, and the vast
> >> >> > majority of the sections are fully populated, you could hack something
> >> >> > in.  We could, for instance, have a global list that's mostly readonly
> >> >> > which tells you which sections need to be have their sizes closely
> >> >> > inspected.  That would work OK if, for instance, you only needed to
> >> >> > check a couple of memory sections in the system.  It'll start to suck if
> >> >> > you made the lists very long.
> >> >>
> >> >> Thanks for advise. As I say, I hope Russell accept 16M section.
> >> >>
> >> >
> >> > It seems what I needed was good sleep....
> >> > How about this if 16M section is not acceptable ?
> >> >
> >> > == NOT TESTED AT ALL, EVEN NOT COMPILED ==
> >> >
> >> > register address of mem_section to memmap itself's page struct's pg->private field.
> >> > This means the page is used for memmap of the section.
> >> > Otherwise, the page is used for other purpose and memmap has a hole.
> >>
> >> It's a very good idea. :)
> >> But can this handle case that a page on memmap pages have struct page
> >> descriptor of hole?
> >> I mean one page can include 128 page descriptor(4096 / 32).
> > yes.
> >
> >> In there, 64 page descriptor is valid but remain 64 page descriptor is on hole.
> >> In this case, free_memmap doesn't free the page.
> >
> > yes. but in that case, there are valid page decriptor for 64pages of holes.
> > pfn_valid() should return true but PG_reserved is set.
> > (This is usual behavior.)
> >
> > My intention is that
> >
> >  - When all 128 page descriptors are unused, free_memmap() will free it.
> >   In that case, clear page->private of a page for freed page descriptors.
> >
> >  - When some of page descriptors are used, free_memmap() can't free it
> >   and page->private points to &mem_section. We may have memmap for memory
> >   hole but pfn_valid() is a function to check there is memmap or not.
> >   The bahavior of pfn_valid() is valid.
> >   Anyway, you can't free only half of page.
> 
> Okay. I missed PageReserved.
> Your idea seems to be good. :)
> 
> I looked at pagetypeinfo_showblockcount_print.
> It doesn't check PageReserved. Instead of it, it does ugly memmap_valid_within.
> Can't we remove it and change it with PageReserved?
> 
maybe. but I'm not sure how many archs uses CONFIG_ARCH_HAS_HOLES_MEMORYMODEL.
Because my idea requires to add arch-dependent hook, enhancement of pfn_valid()
happens only when an arch supports it. So, you may need a conservative path.

Anyway, I can't test the patch by myself. So, I pass ball to ARM guys.
Feel free to reuse my idea if you like.
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Thanks,
-Kame