Re: [RFC] Tight check of pfn_valid on sparsemem

[KAMEZAWA Hiroyuki]

On Tue, 13 Jul 2010 08:20:09 +0100
Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx> wrote:

> On Tue, Jul 13, 2010 at 03:04:00PM +0900, Minchan Kim wrote:
> > > __get_user() works with TLB and page table, the vaddr is really mapped or not.
> > > If you got SEGV, __get_user() returns -EFAULT. It works per page granule.
> 
> Not in kernel space.  It works on 1MB sections there.
> 
> Testing whether a page is mapped by __get_user is a hugely expensive
> way to test whether a PFN is valid.

Note: pfn_valid() is for checking "there is memmap". 

> It'd be cheaper to use our flatmem implementation of pfn_valid() instead.
> 
Hmm. IIUC, pfn_valid() succeeds in almost all case if there is a section.
But yes, I'm not familar with ARM. 

I love another idea as I've already shown as preparing _a_ page filled with
0x00004000 and map it into the all holes. PG_reserved will help almost all case
even if it's ugly.

Anyway, sparsemem is designed to be aligned to SECTION_SIZE of memmap.
Please avoid adding new Spaghetti code without proper configs.
Thanks,
-Kame

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