From: Muchun Song <songmuchun@bytedance.com>
To: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Linux Memory Management List <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>,
Michal Hocko <mhocko@suse.com>,
Oscar Salvador <osalvador@suse.de>,
David Hildenbrand <david@redhat.com>,
Matthew Wilcox <willy@infradead.org>,
Naoya Horiguchi <naoya.horiguchi@linux.dev>,
Mina Almasry <almasrymina@google.com>,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [External] [PATCH 3/3] hugetlb: before freeing hugetlb page set dtor to appropriate value
Date: Thu, 15 Jul 2021 11:14:03 +0800 [thread overview]
Message-ID: <CAMZfGtWjZAJfPdow2VgNL9T9jYdLLgekKnnQVtKyzaEwC4FfJQ@mail.gmail.com> (raw)
In-Reply-To: <6c38223b-83f4-ef7d-68d7-27c0f6ae6359@oracle.com>
On Thu, Jul 15, 2021 at 1:39 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 7/14/21 3:57 AM, Muchun Song wrote:
> > On Sat, Jul 10, 2021 at 8:25 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
> >> + /*
> >> + * Very subtle
> >> + *
> >> + * For non-gigantic pages set the destructor to the normal compound
> >> + * page dtor. This is needed in case someone takes an additional
> >> + * temporary ref to the page, and freeing is delayed until they drop
> >> + * their reference.
> >> + *
> >> + * For gigantic pages set the destructor to the null dtor. This
> >> + * destructor will never be called. Before freeing the gigantic
> >> + * page destroy_compound_gigantic_page will turn the compound page
> >> + * into a simple group of pages. After this the destructor does not
> >> + * apply.
> >> + *
> >> + * This handles the case where more than one ref is held when and
> >> + * after update_and_free_page is called.
> >> + */
> >> set_page_refcounted(page);
> >> - set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
> >> + if (hstate_is_gigantic(h))
> >> + set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
> >> + else
> >> + set_compound_page_dtor(page, COMPOUND_PAGE_DTOR);
> >
> > Hi Mike,
> >
> > The race is really subtle. But we also should remove the WARN from
> > free_contig_range, right? Because the refcount of the head page of
> > the gigantic page can be greater than one, but free_contig_range has
> > the following warning.
> >
> > WARN(count != 0, "%lu pages are still in use!\n", count);
> >
>
> I did hit that warning in my testing and thought about removing it.
> However, I decided to keep it because non-hugetlb code also makes use of
> alloc_contig_range/free_contig_range and it might be useful in those
> cases.
>
> My 'guess' is that the warning was added not because of temporary ref
> count increases but rather to point out any code that forgot to drop a
> reference.
Got it. At least this patch looks good to me. So
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
>
> BTW - It is not just the 'head' page which could trigger this warning, but
> any 'tail' page as well. That is because we do not call free_contig_range
> with a compound page, but rather a group of pages all with ref count of
> at least one.
Right.
>
> I'm happy to remove the warning if people do not think it is generally
> useful.
For me, I suggest removing it. If someone has any ideas, please
let us know.
> --
> Mike Kravetz
next prev parent reply other threads:[~2021-07-15 3:14 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-10 0:24 [PATCH 0/3] hugetlb: fix potential ref counting races Mike Kravetz
2021-07-10 0:24 ` [PATCH 1/3] hugetlb: simplify prep_compound_gigantic_page ref count racing code Mike Kravetz
2021-07-13 6:31 ` [External] " Muchun Song
2021-07-10 0:24 ` [PATCH 2/3] hugetlb: drop ref count earlier after page allocation Mike Kravetz
2021-07-10 0:24 ` [PATCH 3/3] hugetlb: before freeing hugetlb page set dtor to appropriate value Mike Kravetz
2021-07-14 10:57 ` [External] " Muchun Song
2021-07-14 17:39 ` Mike Kravetz
2021-07-15 3:14 ` Muchun Song [this message]
2021-07-27 23:41 ` [PATCH 0/3] hugetlb: fix potential ref counting races Mike Kravetz
2021-07-28 4:03 ` Muchun Song
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