From: Oscar Salvador <osalvador@suse.de>
To: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Muchun Song <songmuchun@bytedance.com>,
corbet@lwn.net, tglx@linutronix.de, mingo@redhat.com,
bp@alien8.de, x86@kernel.org, hpa@zytor.com,
dave.hansen@linux.intel.com, luto@kernel.org,
peterz@infradead.org, viro@zeniv.linux.org.uk,
akpm@linux-foundation.org, paulmck@kernel.org,
mchehab+huawei@kernel.org, pawan.kumar.gupta@linux.intel.com,
rdunlap@infradead.org, oneukum@suse.com,
anshuman.khandual@arm.com, jroedel@suse.de,
almasrymina@google.com, rientjes@google.com, willy@infradead.org,
mhocko@suse.com, song.bao.hua@hisilicon.com, david@redhat.com,
naoya.horiguchi@nec.com, joao.m.martins@oracle.com,
duanxiongchun@bytedance.com, linux-doc@vger.kernel.org,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v16 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page
Date: Tue, 23 Feb 2021 10:27:55 +0100 [thread overview]
Message-ID: <20210223092740.GA1998@linux> (raw)
In-Reply-To: <13a5363c-6af4-1e1f-9a18-972ca18278b5@oracle.com>
On Mon, Feb 22, 2021 at 04:00:27PM -0800, Mike Kravetz wrote:
> > -static void update_and_free_page(struct hstate *h, struct page *page)
> > +static int update_and_free_page(struct hstate *h, struct page *page)
> > + __releases(&hugetlb_lock) __acquires(&hugetlb_lock)
> > {
> > int i;
> > + int nid = page_to_nid(page);
> >
> > if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
> > - return;
> > + return 0;
> >
> > h->nr_huge_pages--;
> > - h->nr_huge_pages_node[page_to_nid(page)]--;
> > + h->nr_huge_pages_node[nid]--;
> > + VM_BUG_ON_PAGE(hugetlb_cgroup_from_page(page), page);
> > + VM_BUG_ON_PAGE(hugetlb_cgroup_from_page_rsvd(page), page);
> > + set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
> > + set_page_refcounted(page);
>
> I think you added the set_page_refcounted() because the huge page will
> appear as just a compound page without a reference after dropping the
> hugetlb lock? It might be better to set the reference before modifying
> the destructor. Otherwise, page scanning code could find the non-hugetlb
> compound page with no reference. I could not find any code where this
> would be a problem, but I think it would be safer to set the reference
> first.
But we already had set_page_refcounted() before this patchset there.
Are the worries only because we drop the lock? AFAICS, the "page-scanning"
problem could have happened before as well?
Although, what does page scanning mean in this context?
I am not opposed to move it above, but I would like to understand the concern
here.
>
> > + spin_unlock(&hugetlb_lock);
>
> I really like the way this code is structured. It is much simpler than
> previous versions with retries or workqueue. There is nothing wrong with
> always dropping the lock here. However, I wonder if we should think about
> optimizing for the case where this feature is not enabled and we are not
> freeing a 1G huge page. I suspect this will be the most common case for
> some time, and there is no need to drop the lock in this case.
>
> Please do not change the code based on my comment. I just wanted to bring
> this up for thought.
>
> Is it as simple as checking?
> if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
> spin_unlock(&hugetlb_lock);
>
> /* before return */
> if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
> spin_lock(&hugetlb_lock);
AFAIK, we at least need the hstate_is_gigantic? Comment below says that
free_gigantic_page might block, so we need to drop the lock.
And I am fine with the change overall.
Unless I am missing something, we should not need to drop the lock unless
we need to allocate vmemmap pages (apart from gigantic pages).
>
> > +
> > + 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?
> > +
> > + /*
> > + * If we cannot allocate vmemmap pages, just refuse to free the
> > + * page and put the page back on the hugetlb free list and treat
> > + * as a surplus page.
> > + */
> > + h->surplus_huge_pages++;
> > + h->surplus_huge_pages_node[nid]++;
> > +
> > + /*
> > + * This page is now managed by the hugetlb allocator and has
> > + * no users -- drop the last reference.
> > + */
> > + zeroed = put_page_testzero(page);
> > + VM_BUG_ON_PAGE(!zeroed, page);
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 all for catching corner cases, but not sure how realistic this is.
Moreover, if we __ever__ get there, things can get nasty.
We basically will have an in-use page in the free hugetlb pool, so corruption
will happen. At that point, a plain BUG_ON might be better.
But as I said, I do not think we need that.
I yet need to look further, but what I have seen so far looks good.
--
Oscar Salvador
SUSE L3
next prev parent reply other threads:[~2021-02-23 9:28 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-19 10:49 [PATCH v16 0/9] Free some vmemmap pages of HugeTLB page Muchun Song
2021-02-19 10:49 ` [PATCH v16 1/9] mm: memory_hotplug: factor out bootmem core functions to bootmem_info.c Muchun Song
2021-02-19 10:49 ` [PATCH v16 2/9] mm: hugetlb: introduce a new config HUGETLB_PAGE_FREE_VMEMMAP Muchun Song
2021-02-19 10:49 ` [PATCH v16 3/9] mm: hugetlb: free the vmemmap pages associated with each HugeTLB page Muchun Song
2021-02-19 10:49 ` [PATCH v16 4/9] mm: hugetlb: alloc " Muchun Song
2021-02-19 14:12 ` Michal Hocko
2021-02-20 4:20 ` [External] " Muchun Song
2021-02-22 9:25 ` Michal Hocko
2021-02-22 10:31 ` Muchun Song
2021-02-22 10:50 ` Oscar Salvador
2021-02-23 0:00 ` Mike Kravetz
2021-02-23 5:35 ` [External] " Muchun Song
2021-02-23 9:27 ` Oscar Salvador [this message]
2021-02-23 10:27 ` Muchun Song
2021-02-23 10:50 ` Oscar Salvador
2021-02-23 15:41 ` Oscar Salvador
2021-02-23 22:31 ` Oscar Salvador
2021-02-24 3:47 ` Muchun Song
2021-02-24 8:31 ` Oscar Salvador
2021-02-19 10:49 ` [PATCH v16 5/9] mm: hugetlb: set the PageHWPoison to the raw error page Muchun Song
2021-02-19 10:49 ` [PATCH v16 6/9] mm: hugetlb: add a kernel parameter hugetlb_free_vmemmap Muchun Song
2021-02-19 10:49 ` [PATCH v16 7/9] mm: hugetlb: introduce nr_free_vmemmap_pages in the struct hstate Muchun Song
2021-02-19 10:49 ` [PATCH v16 8/9] mm: hugetlb: gather discrete indexes of tail page Muchun Song
2021-02-19 10:49 ` [PATCH v16 9/9] mm: hugetlb: optimize the code with the help of the compiler Muchun Song
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