linux-ext4 - Re: [PATCH v2 00/18] Fix the DAX-gup mistake

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Date:   Tue, 20 Sep 2022 11:29:49 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     akpm@...ux-foundation.org, Jan Kara <jack@...e.cz>,
        Christoph Hellwig <hch@....de>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        John Hubbard <jhubbard@...dia.com>,
        linux-fsdevel@...r.kernel.org, nvdimm@...ts.linux.dev,
        linux-xfs@...r.kernel.org, linux-mm@...ck.org,
        linux-ext4@...r.kernel.org
Subject: Re: [PATCH v2 00/18] Fix the DAX-gup mistake

On Thu, Sep 15, 2022 at 08:35:08PM -0700, Dan Williams wrote:

> This hackery continues the status of DAX pages as special cases in the
> VM. The thought being carrying the Xarray / mapping infrastructure
> forward still allows for the continuation of the page-less DAX effort.
> Otherwise, the work to convert DAX pages to behave like typical
> vm_normal_page() needs more investigation to untangle transparent huge
> page assumptions.

I see it differently, ZONE_DEVICE by definition is page-based. As long
as DAX is using ZONE_DEVICE it should follow the normal struct page
rules, including proper reference counting everywhere.

By not doing this DAX is causing all ZONE_DEVICE users to suffer
because we haven't really special cased just DAX out of all the other
users.

If there is some kind of non-struct page future, then it will not be
ZONE_DEVICE and it will have its own mechanisms, somehow.

So, we should be systematically stripping away all the half-backed
non-struct page stuff from ZONE_DEVICE as a matter of principle. DAX
included, whatever DAX's future may hold.

The pte bit and the missing refcounting in the page table paths is the
remaining big issue and I hope we fix it. The main problem is that
FS-DAX must create compound pages for the 2M page size.

Jason