From: Michal Hocko <mhocko@suse.com>
To: Muchun Song <songmuchun@bytedance.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>,
Andrew Morton <akpm@linux-foundation.org>,
Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
Andi Kleen <ak@linux.intel.com>,
Linux Memory Management List <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [External] Re: [PATCH v2 3/6] mm: hugetlb: fix a race between freeing and dissolving the page
Date: Thu, 7 Jan 2021 13:38:54 +0100 [thread overview]
Message-ID: <20210107123854.GJ13207@dhcp22.suse.cz> (raw)
In-Reply-To: <CAMZfGtV_k=FxcWfazpuT=3ByXSqK-CH-E3yRQUE_dG6JMnAZeg@mail.gmail.com>
On Thu 07-01-21 19:38:00, Muchun Song wrote:
> On Thu, Jan 7, 2021 at 7:18 PM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Thu 07-01-21 16:53:13, Muchun Song wrote:
> > > On Thu, Jan 7, 2021 at 4:41 PM Michal Hocko <mhocko@suse.com> wrote:
> > > >
> > > > On Thu 07-01-21 13:39:38, Muchun Song wrote:
> > > > > On Thu, Jan 7, 2021 at 12:56 AM Michal Hocko <mhocko@suse.com> wrote:
> > > > > >
> > > > > > On Wed 06-01-21 16:47:36, Muchun Song wrote:
> > > > > > > There is a race condition between __free_huge_page()
> > > > > > > and dissolve_free_huge_page().
> > > > > > >
> > > > > > > CPU0: CPU1:
> > > > > > >
> > > > > > > // page_count(page) == 1
> > > > > > > put_page(page)
> > > > > > > __free_huge_page(page)
> > > > > > > dissolve_free_huge_page(page)
> > > > > > > spin_lock(&hugetlb_lock)
> > > > > > > // PageHuge(page) && !page_count(page)
> > > > > > > update_and_free_page(page)
> > > > > > > // page is freed to the buddy
> > > > > > > spin_unlock(&hugetlb_lock)
> > > > > > > spin_lock(&hugetlb_lock)
> > > > > > > clear_page_huge_active(page)
> > > > > > > enqueue_huge_page(page)
> > > > > > > // It is wrong, the page is already freed
> > > > > > > spin_unlock(&hugetlb_lock)
> > > > > > >
> > > > > > > The race windows is between put_page() and spin_lock() which
> > > > > > > is in the __free_huge_page().
> > > > > >
> > > > > > The race window reall is between put_page and dissolve_free_huge_page.
> > > > > > And the result is that the put_page path would clobber an unrelated page
> > > > > > (either free or already reused page) which is quite serious.
> > > > > > Fortunatelly pages are dissolved very rarely. I believe that user would
> > > > > > require to be privileged to hit this by intention.
> > > > > >
> > > > > > > We should make sure that the page is already on the free list
> > > > > > > when it is dissolved.
> > > > > >
> > > > > > Another option would be to check for PageHuge in __free_huge_page. Have
> > > > > > you considered that rather than add yet another state? The scope of the
> > > > > > spinlock would have to be extended. If that sounds more tricky then can
> > > > > > we check the page->lru in the dissolve path? If the page is still
> > > > > > PageHuge and reference count 0 then there shouldn't be many options
> > > > > > where it can be queued, right?
> > > > >
> > > > > Did you mean that we iterate over the free list to check whether
> > > > > the page is on the free list?
> > > >
> > > > No I meant to check that the page is enqueued which along with ref count
> > > > = 0 should mean it has been released to the pool unless I am missing
> > > > something.
> > >
> > > The page can be on the free list or active list or empty when it
> > > is freed to the pool. How to check whether it is on the free list?
> >
> > As I've said, I might be missing something here. But if the page is
> > freed why does it matter whether it is on a active list or free list
> > from the dissolve operation POV?
>
> As you said "check the page->lru". I have a question.
> How to check the page->lru in the dissolve path?
list_empty?
> BTW, dissolve_free_huge_page aims to free the page
> to buddy allocator. put_page (for HugeTLB page) aims
> to free the page to the hugepage pool.
Right. Can we simply back off in the dissolving path when ref count is
0 && PageHuge() if list_empty(page->lru)? Is there any other scenario
when the all above is true and the page is not being freed?
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2021-01-07 12:39 UTC|newest]
Thread overview: 56+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-06 8:47 [PATCH v2 0/6] Fix some bugs about HugeTLB code Muchun Song
2021-01-06 8:47 ` [PATCH v2 1/6] mm: migrate: do not migrate HugeTLB page whose refcount is one Muchun Song
2021-01-06 16:13 ` Michal Hocko
2021-01-07 2:52 ` [External] " Muchun Song
2021-01-07 11:16 ` Michal Hocko
2021-01-07 11:24 ` Muchun Song
2021-01-07 11:48 ` Michal Hocko
2021-01-06 8:47 ` [PATCH v2 2/6] mm: hugetlbfs: fix cannot migrate the fallocated HugeTLB page Muchun Song
2021-01-06 16:35 ` Michal Hocko
2021-01-06 19:30 ` Mike Kravetz
2021-01-06 20:02 ` Michal Hocko
2021-01-06 21:07 ` Mike Kravetz
2021-01-07 8:36 ` Michal Hocko
2021-01-07 2:58 ` [External] " Muchun Song
2021-01-06 8:47 ` [PATCH v2 3/6] mm: hugetlb: fix a race between freeing and dissolving the page Muchun Song
2021-01-06 16:56 ` Michal Hocko
2021-01-06 20:58 ` Mike Kravetz
2021-01-07 3:08 ` [External] " Muchun Song
2021-01-07 8:40 ` Michal Hocko
2021-01-08 0:52 ` Mike Kravetz
2021-01-08 8:28 ` Michal Hocko
2021-01-07 5:39 ` [External] " Muchun Song
2021-01-07 8:41 ` Michal Hocko
2021-01-07 8:53 ` Muchun Song
2021-01-07 11:18 ` Michal Hocko
2021-01-07 11:38 ` Muchun Song
2021-01-07 12:38 ` Michal Hocko [this message]
2021-01-07 12:59 ` Muchun Song
2021-01-07 14:11 ` Michal Hocko
2021-01-07 15:11 ` Muchun Song
2021-01-08 1:06 ` Mike Kravetz
2021-01-08 2:38 ` Muchun Song
2021-01-08 8:43 ` Michal Hocko
2021-01-08 9:01 ` Muchun Song
2021-01-08 9:31 ` Michal Hocko
2021-01-08 10:08 ` Muchun Song
2021-01-08 11:44 ` Michal Hocko
2021-01-08 11:52 ` Muchun Song
2021-01-08 12:04 ` Michal Hocko
2021-01-08 12:24 ` Muchun Song
2021-01-06 8:47 ` [PATCH v2 4/6] mm: hugetlb: add return -EAGAIN for dissolve_free_huge_page Muchun Song
2021-01-06 17:07 ` Michal Hocko
2021-01-07 3:11 ` [External] " Muchun Song
2021-01-07 8:39 ` Michal Hocko
2021-01-07 9:01 ` Muchun Song
2021-01-07 11:22 ` Michal Hocko
2021-01-06 8:47 ` [PATCH v2 5/6] mm: hugetlb: fix a race between isolating and freeing page Muchun Song
2021-01-06 17:10 ` Michal Hocko
2021-01-06 8:47 ` [PATCH v2 6/6] mm: hugetlb: remove VM_BUG_ON_PAGE from page_huge_active Muchun Song
2021-01-06 17:16 ` Michal Hocko
2021-01-08 22:24 ` Mike Kravetz
2021-01-09 4:07 ` [External] " Muchun Song
2021-01-07 9:30 ` [PATCH v2 0/6] Fix some bugs about HugeTLB code David Hildenbrand
2021-01-07 9:40 ` [External] " Muchun Song
2021-01-07 10:10 ` David Hildenbrand
2021-01-07 10:16 ` Muchun Song
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