Re: [PATCH] dcssblk: Mark DAX broken

[Gerald Schaefer <gerald.schaefer@linux.ibm.com>]

On Tue, 24 Sep 2024 15:59:08 -0700
Dan Williams <dan.j.williams@intel.com> wrote:

> 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.

Thanks, that's great news! Without requiring the extra PTE bit, it should
now finally be possible to add struct pages and ZONE_DEVICE support for
dcssblk.

In the meantime, it is OK to pause the DAX support for dcssblk as you
suggested, and finally remove that ugly CONFIG_FS_DAX_LIMITED. Thanks
for bearing with us for so long!

> 
> For now, place the support behind CONFIG_BROKEN, and remove PFN_SPECIAL
> (dcssblk was the only user).

Ok, I guess that PFN_SPECIAL was there because we had no struct pages for
the DCSS memory. When we come back, with proper ZONE_DEVICE and struct
pages, it should not be needed any more.

And yes, the chance to completely remove pfn_t, after Alistair's series,
is quite impressive and even more motivation than CONFIG_FS_DAX_LIMITED.

> 
> Link: http://lore.kernel.org/cover.9f0e45d52f5cff58807831b6b867084d0b14b61c.1725941415.git-series.apopple@nvidia.com [1]
> Link: http://lore.kernel.org/20210820210318.187742e8@thinkpad/ [2]
> Link: http://lore.kernel.org/4511465a4f8429f45e2ac70d2e65dc5e1df1eb47.1725941415.git-series.apopple@nvidia.com [3]
> Cc: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
> Cc: Heiko Carstens <hca@linux.ibm.com>
> Cc: Vasily Gorbik <gor@linux.ibm.com>
> Cc: Alexander Gordeev <agordeev@linux.ibm.com>
> Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
> Cc: Sven Schnelle <svens@linux.ibm.com>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Alistair Popple <apopple@nvidia.com>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
>  drivers/s390/block/Kconfig   |   12 ++++++++++--
>  drivers/s390/block/dcssblk.c |   26 +++++++++++++++++---------
>  fs/Kconfig                   |    9 +--------
>  fs/dax.c                     |   12 ------------
>  include/linux/pfn_t.h        |   15 ---------------
>  mm/memory.c                  |    2 --
>  mm/memremap.c                |    4 ----
>  7 files changed, 28 insertions(+), 52 deletions(-)

When you also remove the now unused dax_dev definition at the top of
dcssblk_add_store(), as noticed by kernel test robot:

Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>