Re: [PATCH 1/2] mm/compaction: fix invalid free_pfn and compact_cached_free_pfn

[David Rientjes <rientjes@google.com>]

On Mon, 21 Dec 2015, Joonsoo Kim wrote:

> free_pfn and compact_cached_free_pfn are the pointer that remember
> restart position of freepage scanner. When they are reset or invalid,
> we set them to zone_end_pfn because freepage scanner works in reverse
> direction. But, because zone range is defined as [zone_start_pfn,
> zone_end_pfn), zone_end_pfn is invalid to access. Therefore, we should
> not store it to free_pfn and compact_cached_free_pfn. Instead, we need
> to store zone_end_pfn - 1 to them. There is one more thing we should
> consider. Freepage scanner scan reversely by pageblock unit. If free_pfn
> and compact_cached_free_pfn are set to middle of pageblock, it regards
> that sitiation as that it already scans front part of pageblock so we
> lose opportunity to scan there. To fix-up, this patch do round_down()
> to guarantee that reset position will be pageblock aligned.
> 
> Note that thanks to the current pageblock_pfn_to_page() implementation,
> actual access to zone_end_pfn doesn't happen until now. But, following
> patch will change pageblock_pfn_to_page() so this patch is needed
> from now on.
> 
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>

Acked-by: David Rientjes <rientjes@google.com>

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