Re: can we finally kill off CONFIG_FS_DAX_LIMITED

Dan Williams
2021-08-23T20:21:15Z

On Mon, Aug 23, 2021 at 12:47 PM Gerald Schaefer
<gerald.schaefer@linux.ibm.com> wrote:
>
> On Mon, 23 Aug 2021 16:05:46 +0200
> Gerald Schaefer <gerald.schaefer@linux.ibm.com> wrote:
>
> > On Fri, 20 Aug 2021 07:43:40 +0200
> > 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
>
> Oh, we do have PTE_SPECIAL, actually that took away the last free bit
> in the pte. So, if there is a chance that ZONE_DEVICE would depend
> on PTE_SPECIAL instead of PTE_DEVMAP, we might be back in the game
> and get rid of that CONFIG_FS_DAX_LIMITED.

So PTE_DEVMAP is primarily there to coordinate the
get_user_pages_fast() path, and even there it's usage can be
eliminated in favor of PTE_SPECIAL. I started that effort [1], but
need to rebase on new notify_failure infrastructure coming from Ruan
[2]. So I think you are not in the critical path until I can get the
PTE_DEVMAP requirement out of your way.

[1]: https://lore.kernel.org/r/161604050866.1463742.7759521510383551055.stgit@dwillia2-desk3.amr.corp.intel.com

[2]: https://lore.kernel.org/r/20210730085245.3069812-1-ruansy.fnst@fujitsu.com