Re: [PATCH 3/3] mm/cma: always check which page cause allocation failure

[Vlastimil Babka]

On 11/13/2015 03:23 AM, Joonsoo Kim wrote:
> Now, we have tracepoint in test_pages_isolated() to notify
> pfn which cannot be isolated. But, in alloc_contig_range(),
> some error path doesn't call test_pages_isolated() so it's still
> hard to know exact pfn that causes allocation failure.
>
> This patch change this situation by calling test_pages_isolated()
> in almost error path. In allocation failure case, some overhead
> is added by this change, but, allocation failure is really rare
> event so it would not matter.
>
> In fatal signal pending case, we don't call test_pages_isolated()
> because this failure is intentional one.
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> ---
>   mm/page_alloc.c | 10 +++++++---
>   1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index d89960d..e78d78f 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6756,8 +6756,12 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>   	if (ret)
>   		return ret;
>
> +	/*
> +	 * In case of -EBUSY, we'd like to know which page causes problem.
> +	 * So, just fall through. We will check it in test_pages_isolated().
> +	 */
>   	ret = __alloc_contig_migrate_range(&cc, start, end);
> -	if (ret)
> +	if (ret && ret != -EBUSY)
>   		goto done;
>
>   	/*
> @@ -6784,8 +6788,8 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>   	outer_start = start;
>   	while (!PageBuddy(pfn_to_page(outer_start))) {
>   		if (++order >= MAX_ORDER) {
> -			ret = -EBUSY;
> -			goto done;
> +			outer_start = start;
> +			break;
>   		}
>   		outer_start &= ~0UL << order;
>   	}

Ugh isn't this crazy loop broken? Shouldn't it test that the buddy it 
finds has order high enough? e.g.:
  buddy = pfn_to_page(outer_start)
  outer_start + (1UL << page_order(buddy)) > start

Otherwise you might end up with something like:
- at "start" there's a page that CMA failed to freed
- at "start-1" there's another non-buddy page
- at "start-3" there's an order-1 buddy, so you set outer_start to start-3
- test_pages_isolated() will complain (via the new tracepoint) about pfn 
of start-1, but actually you would like it to complain about pfn of "start"?

So the loop has been broken before your patch, but it didn't matter, 
just potentially wasted some time by picking bogus outer_start. But now 
your tracepoint will give you weird results.


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