Re: [PATCH] Double check memmap is actually valid with a memmap has unexpected holes V2

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 13 May 2009 17:34:48 +0100
Mel Gorman <mel@csn.ul.ie> wrote:

> pfn_valid() is meant to be able to tell if a given PFN has valid memmap
> associated with it or not. In FLATMEM, it is expected that holes always
> have valid memmap as long as there is valid PFNs either side of the hole.
> In SPARSEMEM, it is assumed that a valid section has a memmap for the
> entire section.
> 
> However, ARM and maybe other embedded architectures in the future free
> memmap backing holes to save memory on the assumption the memmap is never
> used. The page_zone linkages are then broken even though pfn_valid()
> returns true. A walker of the full memmap must then do this additional
> check to ensure the memmap they are looking at is sane by making sure the
> zone and PFN linkages are still valid. This is expensive, but walkers of
> the full memmap are extremely rare.
> 
> This was caught before for FLATMEM and hacked around but it hits again for
> SPARSEMEM because the page_zone linkages can look ok where the PFN linkages
> are totally screwed. This looks like a hatchet job but the reality is that
> any clean solution would end up consumning all the memory saved by punching
> these unexpected holes in the memmap. For example, we tried marking the
> memmap within the section invalid but the section size exceeds the size of
> the hole in most cases so pfn_valid() starts returning false where valid
> memmap exists. Shrinking the size of the section would increase memory
> consumption offsetting the gains.
> 
> This patch identifies when an architecture is punching unexpected holes
> in the memmap that the memory model cannot automatically detect and sets
> ARCH_HAS_HOLES_MEMORYMODEL. At the moment, this is restricted to EP93xx
> which is the model sub-architecture this has been reported on but may expand
> later. When set, walkers of the full memmap must call memmap_valid_within()
> for each PFN and passing in what it expects the page and zone to be for
> that PFN. If it finds the linkages to be broken, it assumes the memmap is
> invalid for that PFN.

It's unclear to me whether this patch is needed in 2.6.30 or even
2.6.29 or whatever.

It applies OK to 2.6.28, 2.6.29, current mainline and mmotm, so I'll
just sit tight until I'm told what to do.

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