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

From: "HORIGUCHI NAOYA(堀口　直也)" <naoya.horiguchi@nec.com>
To: Yang Shi <yang.shi@linux.alibaba.com>,
	Mike Kravetz <mike.kravetz@oracle.com>
Cc: Li Xinhai <lixinhai.lxh@gmail.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	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: Thu, 16 Jan 2020 08:07:30 +0000	[thread overview]
Message-ID: <20200116080729.GA19016@hori.linux.bs1.fc.nec.co.jp> (raw)
In-Reply-To: <2d488bd7-0f2e-f691-b1b6-f2f1faec8ab4@linux.alibaba.com>

Hi everyone,

Thank you all for finding and digging the issue.

> Summary
> =======
> It 'looks' like the statement "MPOL_MF_STRICT is ignored on huge page
> mappings." is left over from the original mbind implementation.  When
> the huge page migration support was added, I can not be sure if ignoring
> MPOL_MF_STRICT for huge pages during the verify/isolation phase was
> intentional.  It seems like it was as the return value from
> isolate_huge_page() is ignored.

This summary is totally correct.  I've simply missed considering MPOL_MF_STRICT
flag when implementing hugetlb migration.  As you pointed out, the discrepacy
between the manpage and the code is also due to the lack of updates on the
"MPOL_MF_STRICT is ignored on huge page mappings." statement.

On Wed, Jan 15, 2020 at 01:59:14PM -0800, Yang Shi wrote:
> 
> On 1/15/20 1:45 PM, Mike Kravetz wrote:
> > On 1/15/20 1:30 PM, Yang Shi wrote:
> > > On 1/15/20 1:07 PM, Mike Kravetz wrote:
> > > > What should we do?
> > > > ==================
> > > > 1) Nothing more than optimizations by Li Xinhai.  Behavior that could be
> > > >      seen as conflicting with man page has existed since v3.12 and I am
> > > >      not aware of any complaints.
> > > > 2) In addition to optimizations by Li Xinhai, modify code to truly ignore
> > > >      MPOL_MF_STRICT for huge page mappings.  This would be fairly easy to do
> > > >      after a failure of migrate_pages().  We could simply traverse the list
> > > >      of pages that were not migrated looking for any non-hugetlb page.
> > > I don't think we can do this easily since migrate_pages() would put the migration failed hugetlb pages back to hugepage_activelist so there should not any hugetlb page reside on the pagelist regardless of failure if I read the code correctly.

Although this behavior seems to me not prevent from finding non-hugetlb
pages in migration list, this is a difference in migration behavior between
normal pages and hugepages that might be better to be optimized.
Maybe hugepages failed to migrate should remain in migration list after
migrate_pages() returns and the should be put back via putback_movable_pages().

> > > 
> > You are correct.  I made an assumption without actually looking at the code. :(
> > 
> > > Other than that traversing page list to look for a certain type page doesn't sound scalable to me.
> > > 
> > > > 3) Remove the statement "MPOL_MF_STRICT is ignored on huge page mappings."
> > > >      and modify code accordingly.
> > > > 
> > > > My suggestion would be for 1 or 2.  Thoughts?
> > > By rethinking the history (thanks again for digging into it), it sounds #3 should be more reasonable. It sounds like the behavior was changed since hugetlb migration was added but the man page was not updated to reflect the change.
> > > 
> > Let's hope Naoya comments.  My only concern with #3 is that we will be changing
> > behavior.  I do not think many people (if any) depend on existing behavior.
> > However, you can never be sure.
> 
> Yes, this would change the bahavior, but I don't see why we have to treat
> hugetlb specially nowadays with migration supported.

(Option #1 is good for short term solution, but eventually) I agree with option #3.
We have no reason to handle hugetlb differently about MPOL_MF_STRICT flag.

Thanks,
Naoya Horiguchi