Re: [RFC PATCH 2/8] hugetlb: recompute min_count when dropping hugetlb_lock

[Mike Kravetz <mike.kravetz@oracle.com>]

On 3/22/21 7:07 AM, Michal Hocko wrote:
> On Fri 19-03-21 15:42:03, Mike Kravetz wrote:
>> The routine set_max_huge_pages reduces the number of hugetlb_pages,
>> by calling free_pool_huge_page in a loop.  It does this as long as
>> persistent_huge_pages() is above a calculated min_count value.
>> However, this loop can conditionally drop hugetlb_lock and in some
>> circumstances free_pool_huge_page can drop hugetlb_lock.  If the
>> lock is dropped, counters could change the calculated min_count
>> value may no longer be valid.
> 
> OK, this one looks like a real bug fix introduced by 55f67141a8927.
> Unless I am missing something we could release pages which are reserved
> already.
>  
>> The routine try_to_free_low has the same issue.
>>
>> Recalculate min_count in each loop iteration as hugetlb_lock may have
>> been dropped.
>>
>> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
>> ---
>>  mm/hugetlb.c | 25 +++++++++++++++++++++----
>>  1 file changed, 21 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index d5be25f910e8..c537274c2a38 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -2521,11 +2521,20 @@ static void __init report_hugepages(void)
>>  	}
>>  }
>>  
>> +static inline unsigned long min_hp_count(struct hstate *h, unsigned long count)
>> +{
>> +	unsigned long min_count;
>> +
>> +	min_count = h->resv_huge_pages + h->nr_huge_pages - h->free_huge_pages;
>> +	return max(count, min_count);
> 
> Just out of curiousity, is compiler allowed to inline this piece of code
> and then cache the value? In other words do we need to make these
> READ_ONCE or otherwise enforce the no-caching behavior?

I honestly do not know if the compiler is allowed to do that.  The
assembly code generated by my compiler does not cache the value, but
that does not guarantee anything.  I can add READ_ONCE to make the
function look something like:

static inline unsigned long min_hp_count(struct hstate *h, unsigned long count)
{
	unsigned long min_count;

	min_count = READ_ONCE(h->resv_huge_pages) + READ_ONCE(h->nr_huge_pages)
					- READ_ONCE(h->free_huge_pages);
	return max(count, min_count);
}

-- 
Mike Kravetz