linux-kernel - Re: [PATCH] mm: remove extra ZONE

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Message-ID: <6dff5231-26d5-1aec-0c05-6880cf747642@nvidia.com>
Date:   Tue, 15 Sep 2020 09:39:47 -0700
From:   Ralph Campbell <rcampbell@...dia.com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     Dan Williams <dan.j.williams@...el.com>,
        Linux MM <linux-mm@...ck.org>, <kvm-ppc@...r.kernel.org>,
        <nouveau@...ts.freedesktop.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        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: [PATCH] mm: remove extra ZONE_DEVICE struct page refcount


On 9/15/20 9:29 AM, Christoph Hellwig wrote:
> On Mon, Sep 14, 2020 at 04:53:25PM -0700, Ralph Campbell wrote:
>> Since set_page_refcounted() is defined in mm_interal.h I would have to
>> move the definition to someplace like page_ref.h or have the drivers
>> cal init_page_count() or set_page_count() since get_page() calls
>> VM_BUG_ON_PAGE() if refcount == 0.
>> I'll move set_page_refcounted() since that is what the page allocator
>> uses and seems named for the purpose.
> 
> I don't think any of the three ->page_free instances even cares about
> the page refcount.
> 
Not true. The page_free() callback records the page is free by setting
a bit or putting the page on a free list but when it allocates a free
device private struct page to be used with migrate_vma_setup(), it needs to
increment the refcount.

For the ZONE_DEVICE MEMORY_DEVICE_GENERIC and MEMORY_DEVICE_PCI_P2PDMA
struct pages, I think you are correct because they don't define page_free()
and from what I can see, don't decrement the page refcount to zero.