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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Fri, 27 Aug 2021 14:41:11 -0700
Message-ID: <CAA9_cmeVK9S2e8ECh3dTaNzUgQHC8uo7DBhENgnvoR3s+w-2Mg@mail.gmail.com>
Subject: Re: [GIT PULL] Memory folios for v5.15
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        "Darrick J. Wong" <djwong@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
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On Fri, Aug 27, 2021 at 11:47 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Fri, Aug 27, 2021 at 10:07:16AM -0400, Johannes Weiner wrote:
> > We have the same thoughts in MM and growing memory sizes. The DAX
> > stuff said from the start it won't be built on linear struct page
> > mappings anymore because we expect the memory modules to be too big to
> > manage them with such fine-grained granularity.
>
> Well, I did.  Then I left Intel, and Dan took over.  Now we have a struct
> page for each 4kB of PMEM.  I'm not particularly happy about this change
> of direction.

Page-less DAX left more problems than it solved. Meanwhile,
ZONE_DEVICE has spawned other useful things like peer-to-peer DMA.

I am more encouraged by efforts to make the 'struct page' overhead
disappear, first from Muchun Song for hugetlbfs and recently Joao
Martins for device-dax.  If anything, I think 'struct page' for PMEM /
DAX *strengthens* the case for folios / better mechanisms to reduce
the overhead of tracking 4K pages.