linux-kernel - Re: [PATCH] mm,memory_hotplug: Explicitly pass the head to isolate_huge

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Date:   Wed, 13 Feb 2019 13:33:39 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>, akpm@...ux-foundation.org,
        david@...hat.com, anthony.yznaga@...cle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm,memory_hotplug: Explicitly pass the head to
 isolate_huge_page

On Wed 13-02-19 09:13:14, Oscar Salvador wrote:
> On Tue, Feb 12, 2019 at 04:13:05PM -0800, Mike Kravetz wrote:
> > Well, commit 94310cbcaa3c ("mm/madvise: enable (soft|hard) offline of
> > HugeTLB pages at PGD level") should have allowed migration of gigantic
> > pages.  I believe it was added for 16GB pages on powerpc.  However, due
> > to subsequent changes I suspsect this no longer works.
> 
> I will take a look, I am definitely interested in that.
> Thanks for pointing it out Mike.
> 
> > 
> > > This check doesn't make much sense in principle. Why should we bail out
> > > based on a section size? We are offlining a pfn range. All that we care
> > > about is whether the hugetlb is migrateable.
> > 
> > Yes.  Do note that the do_migrate_range is only called from __offline_pages
> > with a start_pfn that was returned by scan_movable_pages.  scan_movable_pages
> > has the hugepage_migration_supported check for PageHuge pages.  So, it would
> > seem to be redundant to do another check in do_migrate_range.
> 
> Well, the thing is that if the gigantic page does not start at the very beginning
> of the memblock, and we do find migrateable pages before it in scan_movable_pages(),
> the range that we will pass to do_migrate_ranges() will contain the gigantic page.
> So we need the check there to cover that case too, although I agree that the current
> check is misleading.

Why isn't our check in has_unmovable_pages sufficient?
-- 
Michal Hocko
SUSE Labs