Re: [PATCH v4] mm/mempolicy,hugetlb: Checking hstate for hugetlbfs page in vma_migratable

From: Michal Hocko <mhocko@suse.com>
To: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Li Xinhai <lixinhai.lxh@gmail.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	akpm <akpm@linux-foundation.org>,
	Mike Kravetz <mike.kravetz@oracle.com>
Subject: Re: [PATCH v4] mm/mempolicy,hugetlb: Checking hstate for hugetlbfs page in vma_migratable
Date: Mon, 20 Jan 2020 12:32:00 +0100	[thread overview]
Message-ID: <20200120113200.GZ18451@dhcp22.suse.cz> (raw)
In-Reply-To: <b9024406-7e80-3e17-d79f-395c3669b53f@arm.com>

On Mon 20-01-20 14:51:31, Anshuman Khandual wrote:
> 
> 
> On 01/16/2020 08:48 PM, Michal Hocko wrote:
> > On Thu 16-01-20 21:50:34, Li Xinhai wrote:
> >> On 2020-01-16 at 17:56 Michal Hocko wrote:
> >>> On Thu 16-01-20 04:11:25, Li Xinhai wrote:
> >>>> Checking hstate at early phase when isolating page, instead of during
> >>>> unmap and move phase, to avoid useless isolation.
> >>>
> >>> Could you be more specific what you mean by isolation and why does it
> >>> matter? The patch description should really explain _why_ the change is
> >>> needed or desirable. 
> >>
> >> The changelog can be improved:
> >>
> >> vma_migratable() is called to check if pages in vma can be migrated
> >> before go ahead to isolate, unmap and move pages. For hugetlb pages,
> >> hugepage_migration_supported(struct hstate *h) is one factor which
> >> decide if migration is supported. In current code, this function is called
> >> from unmap_and_move_huge_page(), after isolating page has
> >> completed.
> >> This patch checks hstate from vma_migratable() and avoids isolating pages
> >> which are not supported.
> > 
> > This still explains what but not why this is relevant. If by isolating
> > pages you mean isolate_lru_page then this really a noop for hugetlb
> > pages. Or do I still misread your changelog?
> 
> unmap_and_move_hugepage() aborts migrating a HugeTLB page (from the list)
> if it's corresponding hstate does not support migration.

But all architectures support all hugeltb sizes unless I am missing
something. If there is some which doesn't then the changelog should
mention that. I have already asked for runtime effects with no data
provided.

Just to make it clear. I am not objecting to the patch itself. I am
objecting to the very vague justification. The changelog doesn't explain
_why_ do we need to change this. Is it a bug, non-optimal code, pure
code clean up for a more robust code?

-- 
Michal Hocko
SUSE Labs