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

Michal Hocko
2021-03-23T07:49:06Z

On Mon 22-03-21 09:57:14, Mike Kravetz wrote:
> 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.

Yes, what I meant by the annoyance. On the other hand a parallel access
to a global knob mantaining a global resource should be expected to
have some side effects without an external synchronization unless it is
explicitly documented that such an access is synchronized internally.

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

IMHO it would be a bit of a stretch to consider it a real life problem.

> I'll update the commit message to be more clear.

Thanks! Clarification will definitely help.
--
Michal Hocko
SUSE Labs