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Subject: Re: [PATCH 1/2] hugetlb: fix update_and_free_page contig page struct
 assumption
To:     Jason Gunthorpe <jgg@ziepe.ca>,
        Matthew Wilcox <willy@infradead.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Zi Yan <ziy@nvidia.com>, Davidlohr Bueso <dbueso@suse.de>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Oscar Salvador <osalvador@suse.de>,
        Joao Martins <joao.m.martins@oracle.com>,
        stable@vger.kernel.org
References: <20210217184926.33567-1-mike.kravetz@oracle.com>
 <20210217110252.185c7f5cd5a87c3f7b0c0144@linux-foundation.org>
 <20210218144554.GS2858050@casper.infradead.org>
 <20210218172500.GA4718@ziepe.ca>
From:   Mike Kravetz <mike.kravetz@oracle.com>
Message-ID: <cc94992a-0eff-bf5e-d904-b23d5facfca3@oracle.com>
Date:   Thu, 18 Feb 2021 09:34:41 -0800
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On 2/18/21 9:25 AM, 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 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..

I was thinking that we could detect these 'unusual' configurations and only
do the slower page struct walking in those cases.  However, we would need to
do some research to make sure we have taken into account all possible config
options which can produce non-contiguous page structs.  That should have zero
performance impact in the 'normal' cases.

I suppose we could prohibit gigantic pages in these 'unusual' configurations.
It would require some research to see if this 'may' impact someone.
-- 
Mike Kravetz