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

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Message-ID: <20190213081310.zfxwb3svoqsxnuyc@d104.suse.de>
Date:   Wed, 13 Feb 2019 09:13:14 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Michal Hocko <mhocko@...nel.org>, 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 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.

I will think about it.

-- 
Oscar Salvador
SUSE L3