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

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Date:   Thu, 18 Feb 2021 12:27:58 -0500
From:   Zi Yan <ziy@...dia.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
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 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?

>
> 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.


—
Best Regards,
Yan Zi

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