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

From: Michal Hocko <mhocko@suse.com>
To: Mike Kravetz <mike.kravetz@oracle.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Shakeel Butt <shakeelb@google.com>,
	Oscar Salvador <osalvador@suse.de>,
	David Hildenbrand <david@redhat.com>,
	Muchun Song <songmuchun@bytedance.com>,
	David Rientjes <rientjes@google.com>,
	Miaohe Lin <linmiaohe@huawei.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Matthew Wilcox <willy@infradead.org>,
	HORIGUCHI NAOYA <naoya.horiguchi@nec.com>,
	"Aneesh Kumar K . V" <aneesh.kumar@linux.ibm.com>,
	Waiman Long <longman@redhat.com>, Peter Xu <peterx@redhat.com>,
	Mina Almasry <almasrymina@google.com>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [RFC PATCH 1/8] hugetlb: add per-hstate mutex to synchronize user adjustments
Date: Tue, 23 Mar 2021 08:48:12 +0100	[thread overview]
Message-ID: <YFmdPMBKcc858fUg@dhcp22.suse.cz> (raw)
In-Reply-To: <c88f14aa-cff1-a857-e969-fc5d491e8f2d@oracle.com>

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