From: Michal Hocko <mhocko@suse.com>
To: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Muchun Song <songmuchun@bytedance.com>,
akpm@linux-foundation.org, n-horiguchi@ah.jp.nec.com,
ak@linux.intel.com, linux-mm@kvack.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 2/6] mm: hugetlbfs: fix cannot migrate the fallocated HugeTLB page
Date: Thu, 7 Jan 2021 09:36:22 +0100 [thread overview]
Message-ID: <20210107083622.GA13207@dhcp22.suse.cz> (raw)
In-Reply-To: <b9dcdeb8-6994-55f9-1780-794e17589129@oracle.com>
On Wed 06-01-21 13:07:40, Mike Kravetz wrote:
> On 1/6/21 12:02 PM, Michal Hocko wrote:
> > On Wed 06-01-21 11:30:25, Mike Kravetz wrote:
> >> On 1/6/21 8:35 AM, Michal Hocko wrote:
> >>> On Wed 06-01-21 16:47:35, Muchun Song wrote:
> >>>> Because we only can isolate a active page via isolate_huge_page()
> >>>> and hugetlbfs_fallocate() forget to mark it as active, we cannot
> >>>> isolate and migrate those pages.
> >>>
> >>> I've little bit hard time to understand this initially and had to dive
> >>> into the code to make sense of it. I would consider the following
> >>> wording easier to grasp. Feel free to reuse if you like.
> >>> "
> >>> If a new hugetlb page is allocated during fallocate it will not be
> >>> marked as active (set_page_huge_active) which will result in a later
> >>> isolate_huge_page failure when the page migration code would like to
> >>> move that page. Such a failure would be unexpected and wrong.
> >>> "
> >>>
> >>> Now to the fix. I believe that this patch shows that the
> >>> set_page_huge_active is just too subtle. Is there any reason why we
> >>> cannot make all freshly allocated huge pages active by default?
> >>
> >> I looked into that yesterday. The primary issue is in page fault code,
> >> hugetlb_no_page is an example. If page_huge_active is set, then it can
> >> be isolated for migration. So, migration could race with the page fault
> >> and the page could be migrated before being added to the page table of
> >> the faulting task. This was an issue when hugetlb_no_page set_page_huge_active
> >> right after allocating and clearing the huge page. Commit cb6acd01e2e4
> >> moved the set_page_huge_active after adding the page to the page table
> >> to address this issue.
> >
> > Thanks for the clarification. I was not aware of this subtlety. The
> > existing comment is not helping much TBH. I am still digesting the
> > suggested race. The page is new and exclusive and not visible via page
> > tables yet, so the only source of the migration would be pfn based
> > (hotplug, poisoning), right?
>
> That is correct.
>
>
> > Btw. s@set_page_huge_active@set_page_huge_migrateable@ would help
> > readability IMHO. With a comment explaining that this _has_ to be called
> > after the page is fully initialized.
>
> Agree, I will add that as a future enhancement.
Thanks!
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2021-01-07 8:37 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 [this message]
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
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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