linux-kernel - Re: [PATCH v2] mm/hmm: make device private reference counts zero based

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Message-ID: <20201015074423.GC14082@lst.de>
Date:   Thu, 15 Oct 2020 09:44:23 +0200
From:   Christoph Hellwig <hch@....de>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Ralph Campbell <rcampbell@...dia.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>, 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 v2] mm/hmm: make device private reference counts zero
 based

On Mon, Oct 12, 2020 at 02:14:07PM -0700, Dan Williams 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 device private pages, leaving DAX as still being
> > a special case.
> 
> Please no half-step to removing the special casing...

Agreed.  I really like where the refcount changes are heading, but
I don't think this half-step is helpful.