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

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Message-ID: <1fbc3d0c-327a-6a8c-31c9-bb4c43e97b88@nvidia.com>
Date:   Mon, 5 Oct 2020 09:40:02 -0700
From:   Ralph Campbell <rcampbell@...dia.com>
To:     Christoph Hellwig <hch@....de>
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>,
        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 10/1/20 10:59 PM, Christoph Hellwig 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.
>>
>> Signed-off-by: Ralph Campbell <rcampbell@...dia.com>
> 
> Looks good,
> 
> Reviewed-by: Christoph Hellwig <hch@....de>

Thanks for the review.

I still have reservations about making this an official patch.
Did you see the updated cover letter?
Basically, I'm concerned about ZONE_DEVICE struct pages being inserted into
the process page table with a zero reference count with vmf_insert_mixed().
If it is to be a non-zero reference count, then DAX, pmem, and other uses
of ZONE_DEVICE pages need to be changed (or vmf_insert_mixed()) to
inc/dec in appropriate places but I don't feel I know that code well enough
to make those changes.