linux-kernel - Re: [PATCH 1/2] hugetlb: fix update_and_free

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Message-ID: <20210218172500.GA4718@ziepe.ca>
Date:   Thu, 18 Feb 2021 13:25:00 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Zi Yan <ziy@...dia.com>, Davidlohr Bueso <dbueso@...e.de>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Joao Martins <joao.m.martins@...cle.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH 1/2] hugetlb: fix update_and_free_page contig page struct
 assumption

On Thu, Feb 18, 2021 at 02:45:54PM +0000, Matthew Wilcox wrote:
> On Wed, Feb 17, 2021 at 11:02:52AM -0800, Andrew Morton wrote:
> > On Wed, 17 Feb 2021 10:49:25 -0800 Mike Kravetz <mike.kravetz@...cle.com> wrote:
> > > page structs are not guaranteed to be contiguous for gigantic pages.  The
> >
> > June 2014.  That's a long lurk time for a bug.  I wonder if some later
> > commit revealed it.
> 
> I would suggest that gigantic pages have not seen much use.  Certainly
> performance with Intel CPUs on benchmarks that I've been involved with
> showed lower performance with 1GB pages than with 2MB pages until quite
> recently.

I suggested in another thread that maybe it is time to consider
dropping this "feature"

If it has been slightly broken for 7 years it seems a good bet it
isn't actually being used.

The cost to fix GUP to be compatible with this will hurt normal
GUP performance - and again, that nobody has hit this bug in GUP
further suggests the feature isn't used..

Jason