Re: [PATCH] Tight check of pfn_valid on sparsemem - v4

[Minchan Kim]

On Tue, Jul 27, 2010 at 2:55 PM,  <miltonm@xxxxxxx> wrote:
> [Sorry if i missed or added anyone on cc, patchwork.kernel.org  LKML is not
> working and I'm not subscribed to the list ]

Readd them. :)

>
> On Mon Jul 26 2010 about 12:47:37 EST, Christoph Lameter wrote:
>> On Tue, 27 Jul 2010, Minchan Kim wrote:
>>
>> > This patch registers 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.
>
>>
>> > +void mark_valid_memmap(unsigned long start, unsigned long end);
>> > +
>> > +#ifdef CONFIG_ARCH_HAS_HOLES_MEMORYMODEL
>> > +static inline int memmap_valid(unsigned long pfn)
>> > +{
>> > + struct page *page = pfn_to_page(pfn);
>> > + struct page *__pg = virt_to_page(page);
>> > + return page_private(__pg) == (unsigned long)__pg;
>>
>>
>> What if page->private just happens to be the value of the page struct?
>> Even if that is not possible today, someday someone may add new
>> functionality to the kernel where page->pivage == page is used for some
>> reason.
>>
>> Checking for PG_reserved wont work?
>
> I had the same thought and suggest setting it to the memory section block,
> since that is a uniquie value (unlike PG_reserved),

You mean setting pg->private to mem_section address?
I hope I understand your point.

Actually, KAMEZAWA tried it at first version but I changed it.
That's because I want to support this mechanism to ARM FLATMEM.(It
doesn't have mem_section)


>
>> > static inline int pfn_valid(unsigned long pfn)
>> > {
>> > + struct mem_section *ms;
>> > if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
>> > return 0;
>> > - return valid_section(__nr_to_section(pfn_to_section_nr(pfn)));
>> > + ms = __nr_to_section(pfn_to_section_nr(pfn));
>> > + return valid_section(ms) && memmap_valid(pfn);
>
> .. and we already have computed it when we use it so we could pass it as
> a parameter (to both _valid and mark_valid).

I hope this can support FALTMEM which have holes(ex, ARM).

>
> milton
>



-- 
Kind regards,
Minchan Kim
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