Re: can we finally kill off CONFIG_FS_DAX_LIMITED

From: Dan Williams <dan.j.williams@intel.com>
To: Christoph Hellwig <hch@lst.de>
Cc: Heiko Carstens <hca@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	 Christian Borntraeger <borntraeger@de.ibm.com>,
	Linux NVDIMM <nvdimm@lists.linux.dev>,
	 linux-s390 <linux-s390@vger.kernel.org>,
	Joao Martins <joao.m.martins@oracle.com>,
	 Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Subject: Re: can we finally kill off CONFIG_FS_DAX_LIMITED
Date: Fri, 20 Aug 2021 10:42:14 -0700	[thread overview]
Message-ID: <CAPcyv4jpHX4U3XisqVoaMf_qADDzKyDS1wOijCs3JR_ByrXmHA@mail.gmail.com> (raw)
In-Reply-To: <CAPcyv4i5GHUXPCEu4RbD1x_=usTdK2VWqHfvHFEHijDYBg+CLw@mail.gmail.com>

[ fix Gerald's email ]

On Fri, Aug 20, 2021 at 8:41 AM Dan Williams <dan.j.williams@intel.com> wrote:
>
> [ add Gerald and Joao ]
>
> On Thu, Aug 19, 2021 at 10:44 PM Christoph Hellwig <hch@lst.de> wrote:
> >
> > Hi all,
> >
> > looking at the recent ZONE_DEVICE related changes we still have a
> > horrible maze of different code paths.  I already suggested to
> > depend on ARCH_HAS_PTE_SPECIAL for ZONE_DEVICE there, which all modern
> > architectures have anyway.  But the other odd special case is
> > CONFIG_FS_DAX_LIMITED which is just used for the xpram driver.  Does
> > this driver still see use?  If so can we make it behave like the
> > other DAX drivers and require a pgmap?  I think the biggest missing
> > part would be to implement ARCH_HAS_PTE_DEVMAP for s390.
> >
>
> Gerald,
>
> Might you still be looking to help dcssblk get out of FS_DAX_LIMITED
> jail [1]? I recall Martin saying that 'struct page' overhead was
> prohibitive. I don't know if Joao's 'struct page' diet patches could
> help alleviate that at all (would require the filesystem to only
> allocate blocks in large page sizes).
>
> [1]: https://lore.kernel.org/r/20180523205017.0f2bc83e@thinkpad