linux-kernel - Re: [RFC PATCH 2/3] hugetlb: convert page_huge

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Message-ID: <20210115203803.GB3322@localhost.localdomain>
Date:   Fri, 15 Jan 2021 21:38:03 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Michal Hocko <mhocko@...nel.org>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Muchun Song <songmuchun@...edance.com>,
        David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 2/3] hugetlb: convert page_huge_active() to
 HPageMigratable flag

On Fri, Jan 15, 2021 at 09:43:36AM -0800, Mike Kravetz wrote:
> > Before the page_huge_active() in scan_movable_pages() we have the
> > if (!PageHuge(page)) check, but could it be that between that check and
> > the page_huge_active(), the page gets dissolved, and so we are checking
> > a wrong page[1]? Am I making sense? 
> 
> Yes, you are making sense.
> 
> The reason I decided to drop the check is because it does not eliminate the
> race.  Even with that check in page_huge_active, the page could be dissolved
> between that check and check of page[1].  There really is no way to eliminate
> the race without holding a reference to the page (or hugetlb_lock).  That
> check in page_huge_active just shortens the race window.

Yeah, you are right, the race already exists.
Anyway, do_migrate_range should take care of making sure what it is
handling, so I think we are good.

-- 
Oscar Salvador
SUSE L3