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From:   Zi Yan <ziy@nvidia.com>
To:     Jason Gunthorpe <jgg@ziepe.ca>
CC:     Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
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Subject: Re: [PATCH 1/2] hugetlb: fix update_and_free_page contig page struct
 assumption
Date:   Thu, 18 Feb 2021 12:27:58 -0500
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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@oracle.=
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 late=
r
>>> 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 quit=
e
>> recently.
>
> I suggested in another thread that maybe it is time to consider
> dropping this "feature"

You mean dropping gigantic page support in hugetlb?

>
> 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..

A easy fix might be to make gigantic hugetlb page depends on
CONFIG_SPARSEMEM_VMEMMAP, which guarantee all struct pages are contiguous=
=2E


=E2=80=94
Best Regards,
Yan Zi

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