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

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Date:   Thu, 18 Feb 2021 13:32:00 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Zi Yan <ziy@...dia.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        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 12:27:58PM -0500, Zi Yan wrote:
> On 18 Feb 2021, at 12:25, Jason Gunthorpe wrote:
> 
> > 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"
>
> You mean dropping gigantic page support in hugetlb?

No, I mean dropping support for arches that want to do:

   tail_page != head_page + tail_page_nr

because they can't allocate the required page array either virtually
or physically contiguously.

It seems like quite a burden on the core mm for a very niche, and
maybe even non-existant, case. 

It was originally done for PPC, can these PPC systems use VMEMMAP now?

> > 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..
> 
> A easy fix might be to make gigantic hugetlb page depends on
> CONFIG_SPARSEMEM_VMEMMAP, which guarantee all struct pages are contiguous.

Yes, exactly.

Jason