Re: [PATCH] mm, compaction: fix wrong pfn handling in __reset_isolation_pfn()

[Mel Gorman]

On Tue, Oct 08, 2019 at 05:29:15PM +0200, Vlastimil Babka wrote:
> Florian and Dave reported [1] a NULL pointer dereference in
> __reset_isolation_pfn(). While the exact cause is unclear, staring at the code
> revealed two bugs, which might be related.
> 

I think the fix is a good fit. Even if the problem still occurs, it
eliminates an important possibility.

> One bug is that if zone starts in the middle of pageblock, block_page might
> correspond to different pfn than block_pfn, and then the pfn_valid_within()
> checks will check different pfn's than those accessed via struct page. This
> might result in acessing an unitialized page in CONFIG_HOLES_IN_ZONE configs.
> 

s/acessing/accessing/

Aside from HOLES_IN_ZONE, the patch addresses an issue if the start
of the zone is not pageblock-aligned. While this is common, it's not
guaranteed. I don't think this needs to be clarified in the changelog as
your example is valid. I'm commenting in case someone decides not to try
the patch because they feel HOLES_IN_ZONE is required.

> The other bug is that end_page refers to the first page of next pageblock and
> not last page of current pageblock. The online and valid check is then wrong
> and with sections, the while (page < end_page) loop might wander off actual
> struct page arrays.
> 
> [1] https://lore.kernel.org/linux-xfs/87o8z1fvqu.fsf@xxxxxxxxxxxxxxxxx/
> 
> Reported-by: Florian Weimer <fw@xxxxxxxxxxxxx>
> Reported-by: Dave Chinner <david@xxxxxxxxxxxxx>
> Fixes: 6b0868c820ff ("mm/compaction.c: correct zone boundary handling when resetting pageblock skip hints")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Just one minor irrelevant note below.

> ---
>  mm/compaction.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index ce08b39d85d4..672d3c78c6ab 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -270,14 +270,15 @@ __reset_isolation_pfn(struct zone *zone, unsigned long pfn, bool check_source,
>  
>  	/* Ensure the start of the pageblock or zone is online and valid */
>  	block_pfn = pageblock_start_pfn(pfn);
> -	block_page = pfn_to_online_page(max(block_pfn, zone->zone_start_pfn));
> +	block_pfn = max(block_pfn, zone->zone_start_pfn);
> +	block_page = pfn_to_online_page(block_pfn);
>  	if (block_page) {
>  		page = block_page;
>  		pfn = block_pfn;
>  	}
>  
>  	/* Ensure the end of the pageblock or zone is online and valid */
> -	block_pfn += pageblock_nr_pages;
> +	block_pfn = pageblock_end_pfn(pfn) - 1;
>  	block_pfn = min(block_pfn, zone_end_pfn(zone) - 1);
>  	end_page = pfn_to_online_page(block_pfn);
>  	if (!end_page)

This is fine and is definetly fixing a potential issue.

> @@ -303,7 +304,7 @@ __reset_isolation_pfn(struct zone *zone, unsigned long pfn, bool check_source,
>  
>  		page += (1 << PAGE_ALLOC_COSTLY_ORDER);
>  		pfn += (1 << PAGE_ALLOC_COSTLY_ORDER);
> -	} while (page < end_page);
> +	} while (page <= end_page);
>  
>  	return false;
>  }

I think this is also ok as it's appropriate for PFN walkers in general of
this style. However, I think it's unlikely to fix anything given that we
are walking in steps of (1 << PAGE_ALLOC_COSTLY_ORDER) and the final page
is not necessarily aligned on that boundary. Still, it's an improvement.

Thanks

-- 
Mel Gorman
SUSE Labs