Re: [PATCH 2/2] mm/mempolicy: Skip walking HUGETLB vma if MPOL_MF_STRICT is specified alone

From: Yang Shi <yang.shi@linux.alibaba.com>
To: Mike Kravetz <mike.kravetz@oracle.com>,
	Li Xinhai <lixinhai.lxh@gmail.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>
Cc: akpm <akpm@linux-foundation.org>, mhocko <mhocko@suse.com>,
	n-horiguchi <n-horiguchi@ah.jp.nec.com>
Subject: Re: [PATCH 2/2] mm/mempolicy: Skip walking HUGETLB vma if MPOL_MF_STRICT is specified alone
Date: Tue, 14 Jan 2020 21:23:01 -0800	[thread overview]
Message-ID: <253e9110-4ffd-e9ba-feec-48ce899af057@linux.alibaba.com> (raw)
In-Reply-To: <f2a54a74-51f8-1a82-d9f2-54f7652b4dd4@oracle.com>

On 1/14/20 8:28 PM, Mike Kravetz wrote:
> On 1/14/20 5:24 PM, Yang Shi wrote:
>>
>> On 1/14/20 5:07 PM, Mike Kravetz wrote:
>>> On 1/14/20 6:09 AM, Li Xinhai wrote:
>>>> Add cc to
>>>> Yang Shi <yang.shi@linux.alibaba.com>
>>>> Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
>>>> , who has been worked on this part
>>>>
>>>> On 2020-01-14 at 17:16 Li Xinhai wrote:
>>>>> Checking MPOL_MF_STRICT is ignored for HUGETLB vma according to mbind man
>>>>> page:
>>>>>
>>>>> Notes
>>>>> MPOL_MF_STRICT is ignored on huge page mappings.
>>>>>
>>>>> If MPOL_MF_STRICT is specified alone without any MOVE flag, we should
>>>>> indicate, from test_walk, that walking this vma should be skipped even if
>>>>> there are misplaced pages.
>>>>>
>>>>> Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
>>>>> Cc: Michal Hocko <mhocko@suse.com>
>>>>> Cc: Mike Kravetz <mike.kravetz@oracle.com>
>>> I do not necessarily disagree with the change.  However, this has made me
>>> question a couple things:
>>> 1) Why does the man page say MPOL_MF_STRICT is ignored on huge page mappings?
>>>      - Is that leftover from the the days when huge page migration was not
>>>        supported?
>>>      - Is it just because huge page migration is more likely to fail than
>>>        base page migration.
>>> 2) Does the mbind code function properly when unable to migrate a huge page
>>>      MPOL_MF_STRICT is set?  A quick look at the code looks like it returns
>>>      EIO.
>> I don't know the answer about question #1 I didn't dig into the history. The queue_pages_hugetlb() returns 0 unconditionally, I think this is what "MPOL_MF_STRICT is ignored on huge page mappings" means in code.
>>
>> It would return -EIO for base pages or THP as what the manpage describes.
>>
> I was thinking about a migration failure after isolation.  This block of
> code in do_mbind() after queue_pages_range() and mbind_range().
>
>          if (!err) {
>                  int nr_failed = 0;
>
>                  if (!list_empty(&pagelist)) {
>                          WARN_ON_ONCE(flags & MPOL_MF_LAZY);
>                          nr_failed = migrate_pages(&pagelist, new_page, NULL,
>                                  start, MIGRATE_SYNC, MR_MEMPOLICY_MBIND);
>                          if (nr_failed)
>                                  putback_movable_pages(&pagelist);
>                  }
>
>                  if ((ret > 0) || (nr_failed && (flags & MPOL_MF_STRICT)))
>                          err = -EIO;

Hmm.. I agree this part in man page does look ambiguous. We may assume 
"MPOL_MF_STRICT is ignored on huge page mappings." implies if 
MPOL_MF_STRICT is specified alone? If MOVE flag is specified it should 
return -EIO if some pages could not be moved as what the man page describes.

I don't know what the intention was at the first place. We may have to 
dig into the history.

>