Linux-mm Archive on
 help / color / Atom feed
From: "Thomas Hellström (VMware)" <>
To: Michal Hocko <>
	"Thomas Hellstrom" <>,
	"Andrew Morton" <>,
	"Matthew Wilcox (Oracle)" <>,
	"Kirill A. Shutemov" <>,
	"Ralph Campbell" <>,
	"Jérôme Glisse" <>,
	"Christian König" <>
Subject: Re: [PATCH v2 2/2] drm/ttm: Fix vm page protection handling
Date: Wed, 4 Dec 2019 15:16:09 +0100
Message-ID: <> (raw)
In-Reply-To: <>

On 12/4/19 2:52 PM, Michal Hocko wrote:
> On Tue 03-12-19 11:48:53, Thomas Hellström (VMware) wrote:
>> From: Thomas Hellstrom <>
>> TTM graphics buffer objects may, transparently to user-space,  move
>> between IO and system memory. When that happens, all PTEs pointing to the
>> old location are zapped before the move and then faulted in again if
>> needed. When that happens, the page protection caching mode- and
>> encryption bits may change and be different from those of
>> struct vm_area_struct::vm_page_prot.
>> We were using an ugly hack to set the page protection correctly.
>> Fix that and instead use vmf_insert_mixed_prot() and / or
>> vmf_insert_pfn_prot().
>> Also get the default page protection from
>> struct vm_area_struct::vm_page_prot rather than using vm_get_page_prot().
>> This way we catch modifications done by the vm system for drivers that
>> want write-notification.
> So essentially this should have any new side effect on functionality it
> is just making a hacky/ugly code less so?

Functionality is unchanged. The use of a on-stack vma copy was severely 
frowned upon in an earlier thread, which also points to another similar 
example using vmf_insert_pfn_prot().

> In other words what are the
> consequences of having page protection inconsistent from vma's?

During the years, it looks like the caching- and encryption flags of 
vma::vm_page_prot have been largely removed from usage. From what I can 
tell, there are no more places left that can affect TTM. We discussed 
__split_huge_pmd_locked() towards the end of that thread, but that 
doesn't affect TTM even with huge page-table entries.


  reply index

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-03 10:48 [PATCH v2 0/2] mm, drm/ttm: Fix pte insertion with customized protection Thomas Hellström (VMware)
2019-12-03 10:48 ` [PATCH v2 1/2] mm: Add and export vmf_insert_mixed_prot() Thomas Hellström (VMware)
2019-12-04 13:48   ` Michal Hocko
2019-12-03 10:48 ` [PATCH v2 2/2] drm/ttm: Fix vm page protection handling Thomas Hellström (VMware)
2019-12-04 13:52   ` Michal Hocko
2019-12-04 14:16     ` Thomas Hellström (VMware) [this message]
2019-12-04 14:35       ` Michal Hocko
2019-12-04 14:36         ` Thomas Hellström (VMware)
2019-12-04 14:42           ` Michal Hocko
2019-12-04 15:19             ` Thomas Hellström (VMware)
2019-12-04 15:26               ` Michal Hocko
2019-12-04 15:29                 ` Thomas Hellström (VMware)

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-mm Archive on

Archives are clonable:
	git clone --mirror linux-mm/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-mm linux-mm/ \
	public-inbox-index linux-mm

Example config snippet for mirrors

Newsgroup available over NNTP:

AGPL code for this site: git clone