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

From: Minchan Kim
Date: Wed Jul 28 2010 - 11:56:33 EST

On Wed, Jul 28, 2010 at 10:14:51AM -0500, Christoph Lameter wrote:
> On Wed, 28 Jul 2010, Minchan Kim wrote:
> > static inline int memmap_valid(unsigned long pfn)
> > {
> > struct page *page = pfn_to_page(pfn);
> > struct page *__pg = virt_to_page(page);
> Does that work both for vmemmap and real mmapping?

When Kame suggested this idea, he doesn't consider vmemmap model.
(He prevent this featur's enabling by config !SPARSEMEM_VMEMMAP)

bool "allow holes in sparsemem's memmap"
default n

When I change it with ARCH_HAS_HOLES_MEMORYMODEL, it was my mistake.

I wonder whether we supports VMEMMAP.
That's because hole problem of sparsemem is specific on ARM.
ARM forks uses it for saving memory space but VMEMMAP does use more memory.
I think it's irony.

> > return page_private(__pg) == MAGIC_MEMMAP && PageReserved(__pg);
> > }
> Problem is that pages may be allocated for the mmap from a variety of
> places. The pages in mmap_init_zone() and allocated during boot may have
> PageReserved set whereas the page allocated via vmemmap_alloc_block() have
> PageReserved cleared since they came from the page allocator.
> You need to have consistent use of PageReserved in page structs for the
> mmap in order to do this properly.

Yes if we supports both model.

> Simplest scheme would be to clear PageReserved() in all page struct
> associated with valid pages and clear those for page structs that do not
> refer to valid pages.

I can't understand your words.
Clear PG_resereved in valid pages and invalid pages both?

I guess your code look like that clear PG_revered on valid memmap
but set PG_reserved on invalid memmap.

invalid memmap pages will be freed by free_memmap and will be used
on any place. How do we make sure it has PG_reserved?

Maybe I don't understand your point.

> Then
> mmap_valid = !PageReserved(xxx(pfn_to_page(pfn))

Kind regards,
Minchan Kim
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