Re: [RFC][PATCH] overwride page->mapping [0/3] intro

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

On Wed, 12 Sep 2007 14:12:14 +0530
Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

> KAMEZAWA Hiroyuki wrote:
> > In general, we cannot inclease size of 'struct page'. So, overriding and
> > adding prural meanings to page struct's member is done in many situation.
> > 
> 
> Hi, Kamezawa,
> 
> We discussed the struct page size issue at VM summit. If I remember
> correctly, Linus suggested that we consider using pfn's instead of
> pointers for pointer members in struct page.
> 
Hmm, define something like this ?
==
#define page_cointainer_for_pfn(pfn)      some_routine
==

> > But to do some kind of precise VM mamangement, page struct itself seems to be
> > too small. This patchset overrides page->mapping and add on-demand page
> > information.
> > 
> > like this:
> > 
> > ==
> > page->mapping points to address_space or anon_vma or mapping_info
> > 
> 
> Could you elaborate a little here, on what is the basis to decide
> what page->mapping should point to?

- page->mapping & 0x3 == 0   -> address_space
- page->mapping & 0x3 == 0x2 -> mapping_info (points to address_space)
- page->mapping & 0x3 == 0x1 -> anon_vma
- page->mapping & 0x3 == 0x3 -> mapping_info (points to anon_vma)



> 
> > mapping_info is strucutured as 
> > 
> > struct mapping_info {
> > 	union {
> > 		anon_vma;
> > 		address_space;
> > 	};
> > 	/* Additional Information to this page */
> > };
> > 
> > ==
> > This works based on "adding page->mapping interface" patch set, I posted.
> > 
> > My main target is move page_container information to this mapping_info.
> > By this, we can avoid increasing size of struct page when container is used.
> > 
> 
> I am not against this goal, but wouldn't we end up with too many
> dereferences to get to the container?
> i.e, page->mapping->page_container->mem_container.
> 
I'm now thinling like this.
==
>From : page->container -> mem_contnainer.
To   : page->mapping_info.container -> mem_container
==                     ^^^^^


Thanks,
-Kame

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