Re: [PATCH 1/3] mm: consider zone which is not fully populated to have holes

[Vlastimil Babka <vbabka@suse.cz>]

On 04/15/2017 02:17 PM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> __pageblock_pfn_to_page has two users currently, set_zone_contiguous
> which checks whether the given zone contains holes and
> pageblock_pfn_to_page which then carefully returns a first valid
> page from the given pfn range for the given zone. This doesn't handle
> zones which are not fully populated though. Memory pageblocks can be
> offlined or might not have been onlined yet. In such a case the zone
> should be considered to have holes otherwise pfn walkers can touch
> and play with offline pages.
> 
> Current callers of pageblock_pfn_to_page in compaction seem to work
> properly right now because they only isolate PageBuddy
> (isolate_freepages_block) or PageLRU resp. __PageMovable
> (isolate_migratepages_block) which will be always false for these pages.
> It would be safer to skip these pages altogether, though. In order
> to do that let's check PageReserved in __pageblock_pfn_to_page because
> offline pages are reserved.

My issue with this is that PageReserved can be also set for other
reasons than offlined block, e.g. by a random driver. So there are two
suboptimal scenarios:

- PageReserved is set on some page in the middle of pageblock. It won't
be detected by this patch. This violates the "it would be safer" argument.
- PageReserved is set on just the first (few) page(s) and because of
this patch, we skip it completely and won't compact the rest of it.

So if we decide we really need to check PageReserved to ensure safety,
then we have to check it on each page. But I hope the existing criteria
in compaction scanners are sufficient. Unless the semantic is that if
somebody sets PageReserved, he's free to repurpose the rest of flags at
his will (IMHO that's not the case).

The pageblock-level check them becomes a performance optimization so
when there's an "offline hole", compaction won't iterate it page by
page. But the downside is the false positive resulting in skipping whole
pageblock due to single page.
I guess it's uncommon for a longlived offline holes to exist, so we
could simply just drop this?

> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>  mm/page_alloc.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 0cacba69ab04..dcbbcfdda60e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1351,6 +1351,8 @@ struct page *__pageblock_pfn_to_page(unsigned long start_pfn,
>  		return NULL;
>  
>  	start_page = pfn_to_page(start_pfn);
> +	if (PageReserved(start_page))
> +		return NULL;
>  
>  	if (page_zone(start_page) != zone)
>  		return NULL;
>