Re: can we finally kill off CONFIG_FS_DAX_LIMITED

Dan Williams
2021-08-24T14:53:35Z

On Tue, Aug 24, 2021 at 7:10 AM Joao Martins <joao.m.martins@oracle.com> wrote:
>
>
>
> On 8/23/21 9:21 PM, Dan Williams wrote:
> > 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.
> >
>
> Isn't the implicit case that PTE_SPECIAL means that you
> aren't supposed to get a struct page back? The gup path bails out on
> pte_special() case. And in the fact in this thread that you quote:
>
> > [1]: https://lore.kernel.org/r/161604050866.1463742.7759521510383551055.stgit@dwillia2-desk3.amr.corp.intel.com
>
> (...) we were speaking about[1.1] using that same special bit to block
> longterm gup for fs-dax (while allowing it device-dax which does support it).
>
> [1.1] https://lore.kernel.org/nvdimm/a8c41028-c7f5-9b93-4721-b8ddcf2427da@oracle.com/
>
> Or maybe that's what you mean for this particular case of FS_DAX_LIMITED. Most _special*()
> cases in mm match _devmap*() as far I've experimented in the past with PMD/PUD and dax
> (prior to [1.1]).
>
> I am just wondering would you differentiate the case where you have metadata for the
> !FS_DAX_LIMITED case in {gup,gup_fast} path in light of removing PTE_DEVMAP. I would have
> thought of checking that a pgmap exists for the pfn (without grabbing a ref to it).

So I should clarify, I'm not proposing removing PTE_DEVMAP, I'm
proposing relaxing its need for architectures that can not afford the
PTE bit. Those architectures would miss out on get_user_pages_fast()
for devmap pages. Then, once PTE_SPECIAL kicks get_user_pages() to the
slow path, get_dev_pagemap() is used to detect devmap pages.