Re: virtual memmap sparsity: Dealing with fragmented MAX_ORDER blocks

[Andy Whitcroft <apw@shadowen.org>]

Christoph Lameter wrote:
> On Mon, 25 Sep 2006, Christoph Lameter wrote:
> 
>>    Looking through the i386 commands I see a VERR mnemonic that
>>    I guess will do what you need on i386 and x86_64 in order to do
>>    what we need without a page table walk.
> 
> I think I guessed wrong VERR does something with segments???
> 
> We could sidestep the issue by not marking the huge page 
> non present but pointing it to a pte page with all pointers to the 
> zero page.

Well we'd really want it to be a page of page*'s marked as in
PG_reserved and probabally in an invalid zone or some such to prevent
them coelesing with logically adjacent buddies.

> 
> All page flags will be cleared of the page structs in the zero page and 
> thus we cannot reference an invalid address. Therefore:
> 
> static inline int page_is_buddy(struct page *page, struct page *buddy,
>                                                                 int order)
> {
> #ifdef CONFIG_HOLES_IN_ZONE
>         if (!pfn_valid(page_to_pfn(buddy)))
>                 return 0;
> #endif
> 
>         if (page_zone_id(page) != page_zone_id(buddy))
>                 return 0;
> 
>         if (PageBuddy(buddy) && page_order(buddy) == order) {
>                 BUG_ON(page_count(buddy) != 0);
>                 return 1;
>         }
>         return 0;
> }
> 
> can become
> 
> static inline int page_is_buddy(struct page *page, struct page *buddy,
>                                                                 int order)
> {
>         if (page_zone_id(page) != page_zone_id(buddy))
>                 return 0;
> 
>         if (PageBuddy(buddy) && page_order(buddy) == order) {
>                 BUG_ON(page_count(buddy) != 0);
>                 return 1;
>         }
>         return 0;
> }
> 
> for all cases.
> 
> Also note that page_zone_id(page) no longer needs to be using a lookup of
> page->flags. We just need to insure that both pages are in the same 
> MAX_ORDER group. For that to be true the upper portion of the addresses
> must match.
> 
> int page_zone_id(struct page *page)
> {
> 	return pfn_page >> MAX_ORDER;
> }

I don't think this is correct.  Zones are not guarenteed to be max order
aligned, and we cannot allow pages in different zones be coelesced.
Indeed, as often they are properly aligned we looked at taking this test
out very recently.  The complexity and risk was deemed to great.  It is
after all a simple check against a cache line which is about to be used
and checked in the very next statement.  So the cost is very small.

-apw

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>