linux-kernel - Re: [GIT PULL] Memory folios for v5.15

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Date:   Fri, 27 Aug 2021 22:49:50 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] Memory folios for v5.15

On Fri, Aug 27, 2021 at 02:41:11PM -0700, Dan Williams wrote:
> On Fri, Aug 27, 2021 at 11:47 AM Matthew Wilcox <willy@...radead.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.

ZONE_DEVICE has created more problems than it solved.  Pageless memory
is a concept which still needs to be supported, and we could have made
a start on that five years ago.  Instead you opted for the expeditious
solution.