Re: [External] Re: [PATCH v16 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

From: Oscar Salvador <osalvador@suse.de>
To: Muchun Song <songmuchun@bytedance.com>
Cc: "Mike Kravetz" <mike.kravetz@oracle.com>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com,
	dave.hansen@linux.intel.com, luto@kernel.org,
	"Peter Zijlstra" <peterz@infradead.org>,
	viro@zeniv.linux.org.uk,
	"Andrew Morton" <akpm@linux-foundation.org>,
	paulmck@kernel.org, mchehab+huawei@kernel.org,
	pawan.kumar.gupta@linux.intel.com,
	"Randy Dunlap" <rdunlap@infradead.org>,
	oneukum@suse.com, anshuman.khandual@arm.com, jroedel@suse.de,
	"Mina Almasry" <almasrymina@google.com>,
	"David Rientjes" <rientjes@google.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Michal Hocko" <mhocko@suse.com>,
	"Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>,
	"David Hildenbrand" <david@redhat.com>,
	"HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>,
	"Joao Martins" <joao.m.martins@oracle.com>,
	"Xiongchun duan" <duanxiongchun@bytedance.com>,
	linux-doc@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
	"Linux Memory Management List" <linux-mm@kvack.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [External] Re: [PATCH v16 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page
Date: Tue, 23 Feb 2021 11:50:05 +0100	[thread overview]
Message-ID: <20210223104957.GA3844@linux> (raw)
In-Reply-To: <CAMZfGtVRSBkKe=tKAKLY8dp_hywotq3xL+EJZNjXuSKt3HK3bQ@mail.gmail.com>

On Tue, Feb 23, 2021 at 06:27:07PM +0800, Muchun Song wrote:
> > > > +
> > > > +   if (alloc_huge_page_vmemmap(h, page)) {
> > > > +           int zeroed;
> > > > +
> > > > +           spin_lock(&hugetlb_lock);
> > > > +           INIT_LIST_HEAD(&page->lru);
> > > > +           set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
> > > > +           h->nr_huge_pages++;
> > > > +           h->nr_huge_pages_node[nid]++;
> >
> > I think prep_new_huge_page() does this for us?
> 
> Actually, there are some differences. e.g. prep_new_huge_page()
> will reset hugetlb cgroup and ClearHPageFreed, but we do not need
> them here. And prep_new_huge_page will acquire and release
> the hugetlb_lock. But here we also need hold the lock to update
> the surplus counter and enqueue the page to the free list.
> So I do not think reuse prep_new_huge_page is a good idea.

I see, I missed that.

> > Can this actually happen? AFAIK, page landed in update_and_free_page should be
> > zero refcounted, then we increase the reference, and I cannot see how the
> > reference might have changed in the meantime.
> 
> I am not sure whether other modules get the page and then put the
> page. I see gather_surplus_pages does the same thing. So I copied
> from there. I try to look at the memory_failure routine.
> 
> 
> CPU0:                           CPU1:
>                                 set_compound_page_dtor(HUGETLB_PAGE_DTOR);
> memory_failure_hugetlb
>   get_hwpoison_page
>     __get_hwpoison_page
>       get_page_unless_zero
>                                 put_page_testzero()
> 
> Maybe this can happen. But it is a very corner case. If we want to
> deal with this. We can put_page_testzero() first and then
> set_compound_page_dtor(HUGETLB_PAGE_DTOR).

I have to check further, but it looks like this could actually happen.
Handling this with VM_BUG_ON is wrong, because memory_failure/soft_offline are
entitled to increase the refcount of the page.

AFAICS,

 CPU0:                                    CPU1:
                                          set_compound_page_dtor(HUGETLB_PAGE_DTOR);
 memory_failure_hugetlb
   get_hwpoison_page
     __get_hwpoison_page
       get_page_unless_zero
                                          put_page_testzero()
        identify_page_state
         me_huge_page

I think we can reach me_huge_page with either refcount = 1 or refcount =2,
depending whether put_page_testzero has been issued.

For now, I would not re-enqueue the page if put_page_testzero == false.
I have to see how this can be handled gracefully.

-- 
Oscar Salvador
SUSE L3