From: David Hildenbrand <david@redhat.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>,
mhocko@suse.cz, Linux Memory Management List <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>,
Yang Shi <shy828301@gmail.com>
Subject: Re: [External] Re: [PATCH v3 1/6] mm: migrate: do not migrate HugeTLB page whose refcount is one
Date: Tue, 12 Jan 2021 15:28:25 +0100 [thread overview]
Message-ID: <3386dc6d-5f68-c1e3-ba27-d0e95364aa3e@redhat.com> (raw)
In-Reply-To: <CAMZfGtXfA6JA3eYLhT8YGLJJocopix_mp7uSDPfcmooFEE7xNA@mail.gmail.com>
On 12.01.21 15:17, Muchun Song wrote:
> On Tue, Jan 12, 2021 at 9:51 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 12.01.21 14:40, Muchun Song wrote:
>>> On Tue, Jan 12, 2021 at 7:11 PM David Hildenbrand <david@redhat.com> wrote:
>>>>
>>>> On 12.01.21 12:00, David Hildenbrand wrote:
>>>>> On 10.01.21 13:40, Muchun Song wrote:
>>>>>> If the refcount is one when it is migrated, it means that the page
>>>>>> was freed from under us. So we are done and do not need to migrate.
>>>>>>
>>>>>> This optimization is consistent with the regular pages, just like
>>>>>> unmap_and_move() does.
>>>>>>
>>>>>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>>>>>> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
>>>>>> Acked-by: Yang Shi <shy828301@gmail.com>
>>>>>> ---
>>>>>> mm/migrate.c | 6 ++++++
>>>>>> 1 file changed, 6 insertions(+)
>>>>>>
>>>>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>>>>> index 4385f2fb5d18..a6631c4eb6a6 100644
>>>>>> --- a/mm/migrate.c
>>>>>> +++ b/mm/migrate.c
>>>>>> @@ -1279,6 +1279,12 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
>>>>>> return -ENOSYS;
>>>>>> }
>>>>>>
>>>>>> + if (page_count(hpage) == 1) {
>>>>>> + /* page was freed from under us. So we are done. */
>>>>>> + putback_active_hugepage(hpage);
>>>>>> + return MIGRATEPAGE_SUCCESS;
>>>>>> + }
>>>>>> +
>>>>>> new_hpage = get_new_page(hpage, private);
>>>>>> if (!new_hpage)
>>>>>> return -ENOMEM;
>>>>>>
>>>>>
>>>>> Question: What if called via alloc_contig_range() where we even want to
>>>>> "migrate" free pages, meaning, relocate it?
>>>>>
>>>>
>>>> To be more precise:
>>>>
>>>> a) We don't have dissolve_free_huge_pages() calls on the
>>>> alloc_contig_range() path. So we *need* migration IIUC.
>>>
>>> Without this patch, if you want to migrate a HUgeTLB page,
>>> the page is freed to the hugepage pool. With this patch,
>>> the page is also freed to the hugepage pool.
>>> I didn't see any different. I am missing something?
>>
>> I am definitely not an expert on hugetlb pools, that's why I am asking.
>>
>> Isn't it, that with your code, no new page is allocated - so
>> dissolve_free_huge_pages() might just refuse to dissolve due to
>> reservations, bailing out, no?
>
> Without this patch, the new page can be allocated from the
> hugepage pool. The dissolve_free_huge_pages() also
> can refuse to dissolve due to reservations. Right?
Oh, you mean the migration target might be coming from the pool? I guess
yes, looking at alloc_migration_target()->alloc_huge_page_nodemask().
In that case, yes, I think we run into a similar issue already.
Instead of trying to allocate new huge pages in
dissolve_free_huge_pages() to "relocate free pages", we bail out.
This all feels kind of wrong. After we migrated a huge page we should
free it back to the buddy, so most of our machinery just keeps working
without caring about free huge pages.
I can see how your patch will not change the current (IMHO broken) behavior.
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2021-01-12 14:30 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-10 12:40 [PATCH v3 0/6] Fix some bugs about HugeTLB code Muchun Song
2021-01-10 12:40 ` [PATCH v3 1/6] mm: migrate: do not migrate HugeTLB page whose refcount is one Muchun Song
2021-01-12 9:42 ` Michal Hocko
2021-01-12 9:43 ` [External] " Muchun Song
2021-01-12 11:00 ` David Hildenbrand
2021-01-12 11:11 ` David Hildenbrand
2021-01-12 11:27 ` Michal Hocko
2021-01-12 11:34 ` David Hildenbrand
2021-01-12 12:16 ` Michal Hocko
2021-01-12 14:23 ` Michal Hocko
2021-01-12 14:41 ` David Hildenbrand
2021-01-12 14:53 ` Michal Hocko
2021-01-12 20:12 ` Mike Kravetz
2021-01-12 13:40 ` [External] " Muchun Song
2021-01-12 13:51 ` David Hildenbrand
2021-01-12 14:17 ` Muchun Song
2021-01-12 14:28 ` David Hildenbrand [this message]
2021-01-12 14:59 ` Muchun Song
2021-01-10 12:40 ` [PATCH v3 2/6] mm: hugetlbfs: fix cannot migrate the fallocated HugeTLB page Muchun Song
2021-01-11 23:04 ` Mike Kravetz
2021-01-12 9:45 ` Michal Hocko
2021-01-10 12:40 ` [PATCH v3 3/6] mm: hugetlb: fix a race between freeing and dissolving the page Muchun Song
2021-01-12 1:06 ` Mike Kravetz
2021-01-12 10:02 ` Michal Hocko
2021-01-12 10:13 ` [External] " Muchun Song
2021-01-12 11:17 ` Michal Hocko
2021-01-12 11:43 ` Muchun Song
2021-01-12 12:37 ` Michal Hocko
2021-01-12 15:05 ` Muchun Song
2021-01-10 12:40 ` [PATCH v3 4/6] mm: hugetlb: add return -EAGAIN for dissolve_free_huge_page Muchun Song
2021-01-12 1:20 ` Mike Kravetz
2021-01-12 8:33 ` Michal Hocko
2021-01-12 9:51 ` [External] " Muchun Song
2021-01-12 10:06 ` Michal Hocko
2021-01-12 10:49 ` Muchun Song
2021-01-12 11:11 ` Michal Hocko
2021-01-12 11:34 ` Muchun Song
2021-01-10 12:40 ` [PATCH v3 5/6] mm: hugetlb: fix a race between isolating and freeing page Muchun Song
2021-01-10 12:40 ` [PATCH v3 6/6] mm: hugetlb: remove VM_BUG_ON_PAGE from page_huge_active Muchun Song
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