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

[Balbir Singh <balbir@linux.vnet.ibm.com>]

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.

> 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?

> 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.

> Maybe other men may have other information they want to remember.
> This patch set implements mlock_counter on mapping_info as *exmaple*.
> (About mlock_counter, overriding page->lru may be able to be used.)
> 
> 
> This approach will consume some amount of memory. But I believe this *additional
> information* can be tunred off easily if the user doesn't want this.
> 
> I'm glad if I can get some comments.
> 


I'll review your patchset and respond if I have any comments or
suggestions.


-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL

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