Re: [PATCH 1/1] mm/cma: fix alloc_contig_range ret code/potential leak

[Mike Kravetz]

On 11/22/2017 04:00 AM, Johannes Weiner wrote:
> On Mon, Nov 20, 2017 at 11:39:30AM -0800, Mike Kravetz wrote:
>> If the call __alloc_contig_migrate_range() in alloc_contig_range
>> returns -EBUSY, processing continues so that test_pages_isolated()
>> is called where there is a tracepoint to identify the busy pages.
>> However, it is possible for busy pages to become available between
>> the calls to these two routines.  In this case, the range of pages
>> may be allocated.   Unfortunately, the original return code (ret
>> == -EBUSY) is still set and returned to the caller.  Therefore,
>> the caller believes the pages were not allocated and they are leaked.
>>
>> Update the return code with the value from test_pages_isolated().
>>
>> Fixes: 8ef5849fa8a2 ("mm/cma: always check which page caused allocation failure")
>> Cc: <stable@xxxxxxxxxxxxxxx>
>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> 
> Wow, good catch.
> 
>> ---
>>  mm/page_alloc.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 77e4d3c5c57b..3605ca82fd29 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -7632,10 +7632,10 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>>  	}
>>  
>>  	/* Make sure the range is really isolated. */
>> -	if (test_pages_isolated(outer_start, end, false)) {
>> +	ret = test_pages_isolated(outer_start, end, false);
>> +	if (ret) {
>>  		pr_info_ratelimited("%s: [%lx, %lx) PFNs busy\n",
>>  			__func__, outer_start, end);
>> -		ret = -EBUSY;
>>  		goto done;
> 
> Essentially, an -EBUSY from __alloc_contig_migrate_range() doesn't
> mean anything, and we return 0 if the rest of the operations succeed.
> 
> Since we never plan on returning that particular -EBUSY, would it be
> more robust to reset it right then and there, rather than letting it
> run on in ret for more than a screenful?
> 
> It would also be good to note in that fall-through comment that the
> pages becoming free on their own is a distinct possibility.
> 
> As Michal points out, this is really subtle. It makes sense to make it
> as explicit as possible.

Ok, I thought about zero'ing ret right after the call to
__alloc_contig_migrate_range and return of -EBUSY.  It just didn't look
right to me.  But, you are correct.  We should make this as explicit as
possible.  I will respin the patch as suggested and be sure to include an
explicit comment when setting ret = 0.

-- 
Mike Kravetz