Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages
From: Thomas Hellström (Intel)
Date: Wed Mar 24 2021 - 06:07:16 EST
On 3/24/21 10:58 AM, Daniel Vetter wrote:
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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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.
Yes, I agree. Seems like the special (SW1) is available also for huge
page table entries on x86 AFAICT, although just not implemented.
Otherwise the SW bits appear completely used up.
The PTE size vmf_insert_pfn__xxx functions either insert one of devmap
or special. I think the only users of the huge insert functions apart
form TTM currently insert devmap so we should probably be able to do the
same, and then DRM / TTM wouldn't need to care at all about special or not.
/Thomas
-Daniel