linux-ext4 - Re: [Lsf-pc] [LSF/MM TOPIC] The end of the DAX experiment

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Message-ID: <CAPcyv4jLTdJyTOy715qvBL_j_deiLoBmu_thkUnFKZKMvZL6hA@mail.gmail.com>
Date:   Thu, 14 Feb 2019 11:31:24 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        lsf-pc@...ts.linux-foundation.org,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>
Subject: Re: [Lsf-pc] [LSF/MM TOPIC] The end of the DAX experiment

On Thu, Feb 14, 2019 at 11:10 AM Jerome Glisse <jglisse@...hat.com> wrote:
>
> On Thu, Feb 14, 2019 at 10:25:07AM -0800, Dan Williams wrote:
> > On Thu, Feb 14, 2019 at 5:46 AM Michal Hocko <mhocko@...nel.org> wrote:
> > >
> > > On Wed 06-02-19 13:12:59, Dan Williams wrote:
> > > [...]
> > > > * Userfaultfd for file-backed mappings and DAX
> > >
> > > I assume that other topics are meant to be FS track but this one is MM,
> > > right?
> >
> > Yes, but I think it is the lowest priority of all the noted sub-topics
> > in this proposal. The DAX-reflink discussion, where a given
> > physical-page may need to be mapped into multiple inodes at different
> > offsets, might be more fruitful to have as a joint discussion with MM.
>
> Note that my generic page write protection work can be use for that ie
> having a single page correspond to multiple different mapping with also
> different offset within each mapping. While in my patchset i only solve
> the mapping aliasing issue, the index can be solve in much the same way
> because same thinking apply. Namely that when you work on a file you
> know the mapping and file offset and thus the index and when you work on
> the vma you know the mapping and offset within the vma which translate
> to offset within the file. They are only few places that do not have the
> informations available and those do not care about it.
>
> I am just again working on my struct page mapping patchset as well as
> the generic page write protection that sits on top. I hope to be able
> to post the v2 in couple weeks. You can always look at my posting last
> year to see more details.

Yes, I have that in mind as one of the contenders. However, it's not
clear to me that its a suitable fit for filesystem-reflink. Others
have floated the 'page proxy' idea, so it would be good to discuss the
merits of the general approaches.