linux-kernel - Re: [RFC PATCH v3 2/2] mm: remove extra ZONE

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Message-ID: <751582db-9f03-dfbf-1466-131e051def37@nvidia.com>
Date:   Thu, 8 Oct 2020 09:45:55 -0700
From:   Ralph Campbell <rcampbell@...dia.com>
To:     Ram Pai <linuxram@...ibm.com>
CC:     <linux-mm@...ck.org>, <kvm-ppc@...r.kernel.org>,
        <nouveau@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jerome Glisse <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Alistair Popple <apopple@...dia.com>,
        "Christoph Hellwig" <hch@....de>, Jason Gunthorpe <jgg@...dia.com>,
        Bharata B Rao <bharata@...ux.ibm.com>,
        Zi Yan <ziy@...dia.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Paul Mackerras <paulus@...abs.org>,
        Ben Skeggs <bskeggs@...hat.com>,
        "Andrew Morton" <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH v3 2/2] mm: remove extra ZONE_DEVICE struct page
 refcount


On 10/7/20 10:17 PM, Ram Pai wrote:
> On Thu, Oct 01, 2020 at 11:17:15AM -0700, Ralph Campbell wrote:
>> ZONE_DEVICE struct pages have an extra reference count that complicates the
>> code for put_page() and several places in the kernel that need to check the
>> reference count to see that a page is not being used (gup, compaction,
>> migration, etc.). Clean up the code so the reference count doesn't need to
>> be treated specially for ZONE_DEVICE.
> 
> 
> I was hoping this patch would resolve a page-reference issue that we run
> into at random times while migrating a page to a device page backed by
> secure-memory.
> 
> Unfortunately I continue to see the problem. There is a reference
> held on that page, which fails the migration.
> 
> FYI
> RP

I'm willing to look into it but I would need more information.
Can you give any more details about the conditions when it happens?
It would be great if you have a program that reproduces the problem.