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

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Date:   Tue, 20 Sep 2022 09:50:50 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        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

Jason Gunthorpe wrote:
> 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.

Yes, this is how I see it too. Without serious help from folks that want
to kill struct-page usage with DAX the next step will be dynamic
compound page metadata initialization whenever a PMD entry is installed.