linux-kernel - Re: [PATCH v6 21/26] fs/dax: Properly refcount fs dax pages

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Message-ID: <y2uilkilpl4k5snzxntt4x62z4d3mumxqyog3gy3zrey3jewuz@x3ftslqihoil>
Date: Mon, 10 Feb 2025 10:35:35 +1100
From: Alistair Popple <apopple@...dia.com>
To: Dan Williams <dan.j.williams@...el.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org, 
	alison.schofield@...el.com, lina@...hilina.net, zhang.lyra@...il.com, 
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Subject: Re: [PATCH v6 21/26] fs/dax: Properly refcount fs dax pages

On Thu, Feb 06, 2025 at 09:50:07PM -0800, Dan Williams wrote:
> Alistair Popple wrote:
> > On Mon, Jan 13, 2025 at 07:35:07PM -0800, Dan Williams wrote:
> > > Alistair Popple wrote:
> > 
> > [...]
> > 
> > > ...and here is that aformentioned patch:
> > 
> > This patch is different from what you originally posted here:
> > https://yhbt.net/lore/linux-s390/172721874675.497781.3277495908107141898.stgit@dwillia2-xfh.jf.intel.com/
> > 
> > > -- 8< --
> > > Subject: dcssblk: Mark DAX broken, remove FS_DAX_LIMITED support
> > > 
> > > From: Dan Williams <dan.j.williams@...el.com>
> > > 
> > > The dcssblk driver has long needed special case supoprt to enable
> > > limited dax operation, so called CONFIG_FS_DAX_LIMITED. This mode
> > > works around the incomplete support for ZONE_DEVICE on s390 by forgoing
> > > the ability of dax-mapped pages to support GUP.
> > > 
> > > Now, pending cleanups to fsdax that fix its reference counting [1] depend on
> > > the ability of all dax drivers to supply ZONE_DEVICE pages.
> > > 
> > > To allow that work to move forward, dax support needs to be paused for
> > > dcssblk until ZONE_DEVICE support arrives. That work has been known for
> > > a few years [2], and the removal of "pte_devmap" requirements [3] makes the
> > > conversion easier.
> > > 
> > > For now, place the support behind CONFIG_BROKEN, and remove PFN_SPECIAL
> > > (dcssblk was the only user).
> > 
> > Specifically it no longer removes PFN_SPECIAL. Was this intentional? Or should I
> > really have picked up the original patch from the mailing list?
> 
> I think this patch that only removes the dccsblk usage of PFN_SPECIAL is
> sufficient. Leave the rest to the pfn_t cleanup.

Makes sense. I noticed it when rebaing the pfn_t cleanup because previously it
did remove PFN_SPECIAL so was just wondering if it was intentional. I will add
a patch removing PFN_SPECIAL to the pfn_t/pXX_devmap cleanup series I'm writing
now.