linux-kernel - Re: [PATCH v2 2/5] hugetlb: convert page_huge

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Date:   Wed, 20 Jan 2021 11:00:23 +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>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 2/5] hugetlb: convert page_huge_active()
 HPageMigratable flag

On Wed, Jan 20, 2021 at 10:59:05AM +0100, Oscar Salvador wrote:
> On Tue, Jan 19, 2021 at 05:30:46PM -0800, Mike Kravetz wrote:
> > Use the new hugetlb page specific flag HPageMigratable to replace the
> > page_huge_active interfaces.  By it's name, page_huge_active implied
> > that a huge page was on the active list.  However, that is not really
> > what code checking the flag wanted to know.  It really wanted to determine
> > if the huge page could be migrated.  This happens when the page is actually
> > added the page cache and/or task page table.  This is the reasoning behind
> > the name change.
> > 
> > The VM_BUG_ON_PAGE() calls in the *_huge_active() interfaces are not
> > really necessary as we KNOW the page is a hugetlb page.  Therefore, they
> > are removed.
> > 
> > The routine page_huge_active checked for PageHeadHuge before testing the
> > active bit.  This is unnecessary in the case where we hold a reference or
> > lock and know it is a hugetlb head page.  page_huge_active is also called
> > without holding a reference or lock (scan_movable_pages), and can race with
> > code freeing the page.  The extra check in page_huge_active shortened the
> > race window, but did not prevent the race.  Offline code calling
> > scan_movable_pages already deals with these races, so removing the check
> > is acceptable.  Add comment to racy code.
> > 
> > Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
> 
> Hi Mike,
> 
> This comment addresses both this patch and the next one.
> 
> Instead of putting the SetHPageMigratable flag spread over the
> allocation paths, would it make more sense to place it in
> alloc_huge_page before returning the page?
> Then we could opencode SetHPageMigratableIfSupported right there.

and in putback_active_hugepage.

-- 
Oscar Salvador
SUSE L3