Re: [RFC PATCH 1/8] hugetlb: add per-hstate mutex to synchronize user adjustments

[Mike Kravetz]

On 3/22/21 6:59 AM, Michal Hocko wrote:
> On Fri 19-03-21 15:42:02, Mike Kravetz wrote:
>> The number of hugetlb pages can be adjusted by writing to the
>> sysps/proc files nr_hugepages, nr_hugepages_mempolicy or
>> nr_overcommit_hugepages.  There is nothing to prevent two
>> concurrent modifications via these files.  The underlying routine
>> set_max_huge_pages() makes assumptions that only one occurrence is
>> running at a time.  Specifically, alloc_pool_huge_page uses a
>> hstate specific variable without any synchronization.
> 
> From the above it is not really clear whether the unsynchronized nature
> of set_max_huge_pages is really a problem or a mere annoynce. I suspect
> the later because counters are properly synchronized with the
> hugetlb_lock. It would be great to clarify that.
>  

It is a problem and an annoyance.

The problem is that alloc_pool_huge_page -> for_each_node_mask_to_alloc is
called after dropping the hugetlb lock.  for_each_node_mask_to_alloc
uses the helper hstate_next_node_to_alloc which uses and modifies
h->next_nid_to_alloc.  Worst case would be two instances of set_max_huge_pages
trying to allocate pages on different sets of nodes.  Pages could get
allocated on the wrong nodes.

I really doubt this problem has ever been experienced in practice.
However, when looking at the code in was a real annoyance. :)

I'll update the commit message to be more clear.
-- 
Mike Kravetz