linux-ext4 - Re: [PATCH v2 10/18] fsdax: Manage pgmap references at entry insertion and deletion

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Date:   Wed, 21 Sep 2022 14:38:56 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Dan Williams <dan.j.williams@...el.com>,
        Jason Gunthorpe <jgg@...dia.com>
CC:     <akpm@...ux-foundation.org>, Matthew Wilcox <willy@...radead.org>,
        "Jan Kara" <jack@...e.cz>, "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Hellwig <hch@....de>,
        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 10/18] fsdax: Manage pgmap references at entry
 insertion and deletion

Dan Williams wrote:
> Jason Gunthorpe wrote:
> > On Thu, Sep 15, 2022 at 08:36:07PM -0700, Dan Williams wrote:
> > > The percpu_ref in 'struct dev_pagemap' is used to coordinate active
> > > mappings of device-memory with the device-removal / unbind path. It
> > > enables the semantic that initiating device-removal (or
> > > device-driver-unbind) blocks new mapping and DMA attempts, and waits for
> > > mapping revocation or inflight DMA to complete.
> > 
> > This seems strange to me
> > 
> > The pagemap should be ref'd as long as the filesystem is mounted over
> > the dax. The ref should be incrd when the filesystem is mounted and
> > decrd when it is unmounted.
> > 
> > When the filesystem unmounts it should zap all the mappings (actually
> > I don't think you can even unmount a filesystem while mappings are
> > open) and wait for all page references to go to zero, then put the
> > final pagemap back.
> > 
> > The rule is nothing can touch page->pgmap while page->refcount == 0,
> > and if page->refcount != 0 then page->pgmap must be valid, without any
> > refcounting on the page map itself.
> > 
> > So, why do we need pgmap refcounting all over the place? It seems like
> > it only existed before because of the abuse of the page->refcount?
> 
> Recall that this percpu_ref is mirroring the same function as
> blk_queue_enter() whereby every new request is checking to make sure the
> device is still alive, or whether it has started exiting.
> 
> So pgmap 'live' reference taking in fs/dax.c allows the core to start
> failing fault requests once device teardown has started. It is a 'block
> new, and drain old' semantic.

However this line of questioning has me realizing that I have the
put_dev_pagemap() in the wrong place. It needs to go in
free_zone_device_page(), so that gup extends the lifetime of the device.