Linux-mm Archive on lore.kernel.org
 help / color / Atom feed
From: "Thomas Hellström (VMware)" <thomas_os@shipmail.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	dri-devel@lists.freedesktop.org, pv-drivers@vmware.com,
	linux-graphics-maintainer@vmware.com,
	"Thomas Hellstrom" <thellstrom@vmware.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Matthew Wilcox (Oracle)" <willy@infradead.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	"Ralph Campbell" <rcampbell@nvidia.com>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Christian König" <christian.koenig@amd.com>
Subject: Re: [PATCH v2 2/2] drm/ttm: Fix vm page protection handling
Date: Wed, 4 Dec 2019 16:19:27 +0100
Message-ID: <b7b3ba5a-f625-36bc-d9cf-d537ec60e592@shipmail.org> (raw)
In-Reply-To: <20191204144248.GK25242@dhcp22.suse.cz>

On 12/4/19 3:42 PM, Michal Hocko wrote:
> On Wed 04-12-19 15:36:58, Thomas Hellström (VMware) wrote:
>> On 12/4/19 3:35 PM, Michal Hocko wrote:
>>> On Wed 04-12-19 15:16:09, Thomas Hellström (VMware) wrote:
>>>> 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 <thellstrom@vmware.com>
>>>>>>
>>>>>> 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().
>>>>
>>>> https://lore.kernel.org/lkml/20190905103541.4161-2-thomas_os@shipmail.org/
>>>>
>>>>> 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.
>>> Please state all those details/assumptions you are operating on in the
>>> changelog.
>> Thanks. I'll update the patchset and add that.
> And thinking about that this also begs for a comment in the code to
> explain that some (which?) mappings might have a mismatch and the
> generic code have to be careful. Because as things stand now this seems
> to be really subtle and happen to work _now_ and might break in the future.
> Or what does prevent a generic code to stumble over this discrepancy?

Yes we had that discussion in the thread I pointed to. I initially 
suggested and argued for updating the vma::vm_page_prot using a 
WRITE_ONCE() (we only have the mmap_sem in read mode), there seems to be 
other places in generic code that does the same.

But I was convinced by Andy that this was the right way and also was 
used elsewhere.

(See also 
https://elixir.bootlin.com/linux/latest/source/arch/x86/entry/vdso/vma.c#L116)

I guess to have this properly formulated, what's required is that 
generic code doesn't build page-table entries using vma::vm_page_prot 
for VM_PFNMAP and VM_MIXEDMAP outside of driver control.

/Thomas





  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)
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) [this message]
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:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

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

  git send-email \
    --in-reply-to=b7b3ba5a-f625-36bc-d9cf-d537ec60e592@shipmail.org \
    --to=thomas_os@shipmail.org \
    --cc=akpm@linux-foundation.org \
    --cc=christian.koenig@amd.com \
    --cc=dri-devel@lists.freedesktop.org \
    --cc=jglisse@redhat.com \
    --cc=kirill.shutemov@linux.intel.com \
    --cc=linux-graphics-maintainer@vmware.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=mhocko@kernel.org \
    --cc=pv-drivers@vmware.com \
    --cc=rcampbell@nvidia.com \
    --cc=thellstrom@vmware.com \
    --cc=willy@infradead.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

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

Linux-mm Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-mm/0 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/ https://lore.kernel.org/linux-mm \
		linux-mm@kvack.org
	public-inbox-index linux-mm

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kvack.linux-mm


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git