Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Mar 23, 2021 at 09:42:18PM +0100, Thomas Hellström (Intel) wrote:
> 
> On 3/23/21 8:52 PM, Williams, Dan J wrote:
> > On Sun, 2021-03-21 at 19:45 +0100, Thomas Hellström (Intel) wrote:
> > > TTM sets up huge page-table-entries both to system- and device
> > > memory,
> > > and we don't want gup to assume there are always valid backing struct
> > > pages for these. For PTEs this is handled by setting the pte_special
> > > bit,
> > > but for the huge PUDs and PMDs, we have neither pmd_special nor
> > > pud_special. Normally, huge TTM entries are identified by looking at
> > > vma_is_special_huge(), but fast gup can't do that, so as an
> > > alternative
> > > define _devmap entries for which there are no backing dev_pagemap as
> > > special, update documentation and make huge TTM entries _devmap,
> > > after
> > > verifying that there is no backing dev_pagemap.
> > Please do not abuse p{m,u}d_devmap like this. I'm in the process of
> > removing get_devpagemap() from the gup-fast path [1]. Instead there
> > should be space for p{m,u}d_special in the page table entries (at least
> > for x86-64). So the fix is to remove that old assumption that huge
> > pages can never be special.
> > 
> > [1]:
> > http://lore.kernel.org/r/161604050866.1463742.7759521510383551055.stgit@dwillia2-desk3.amr.corp.intel.com
> > 
> Hmm, yes with that patch it will obviously not work as intended.
> 
> Given that, I think we'll need to disable the TTM huge pages for now until
> we can sort out and agree on using a page table entry bit.

Yeah :-/

I think going full pud/pmd_mkspecial should then also mesh well with
Jason's request to wrap it all up into a vmf_insert_* helper, so at least
it would all look rather pretty in the end.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch