On 3/4/21 6:58 PM, Felix Kuehling wrote:
Am 2021-03-01 um 3:46 a.m. schrieb Thomas Hellström (Intel):
On 3/1/21 9:32 AM, Daniel Vetter wrote:
On Wed, Jan 06, 2021 at 10:01:09PM -0500, Felix Kuehling wrote:
From: Philip Yang <Philip.Yang@xxxxxxx>
Register vram memory as MEMORY_DEVICE_PRIVATE type resource, to
allocate vram backing pages for page migration.
Signed-off-by: Philip Yang <Philip.Yang@xxxxxxx>
Signed-off-by: Felix Kuehling <Felix.Kuehling@xxxxxxx>
So maybe I'm getting this all wrong, but I think that the current ttm
fault code relies on devmap pte entries (especially for hugepte entries)
to stop get_user_pages. But this only works if the pte happens to not
point at a range with devmap pages.
I don't think that's in TTM yet, but the proposed fix, yes (see email
I just sent in another thread),
but only for huge ptes.
This patch here changes that, and so probably breaks this devmap pte
hack
ttm is using?
If I'm not wrong here then I think we need to first fix up the ttm
code to
not use the devmap hack anymore, before a ttm based driver can
register a
dev_pagemap. Also adding Thomas since that just came up in another
discussion.
It doesn't break the ttm devmap hack per se, but it indeed allows gup
to the range registered, but here's where my lack of understanding why
we can't allow gup-ing TTM ptes if there indeed is a backing
struct-page? Because registering MEMORY_DEVICE_PRIVATE implies that,
right?
I wasn't aware that TTM used devmap at all. If it does, what type of
memory does it use?
MEMORY_DEVICE_PRIVATE is like swapped out memory. It cannot be mapped in
the CPU page table. GUP would cause a page fault to swap it back into
system memory. We are looking into use MEMORY_DEVICE_GENERIC for a
future coherent memory architecture, where device memory can be
coherently accessed by the CPU and GPU.
As I understand it, our DEVICE_PRIVATE registration is not tied to an
actual physical address. Thus your devmap registration and our devmap
registration could probably coexist without any conflict. You'll just
have the overhead of two sets of struct pages for the same memory.
Regards,
Felix
Hi, Felix. TTM doesn't use devmap yet, but thinking of using it for
faking pmd_special() which isn't available. That would mean pmd_devmap()
+ no_registered_dev_pagemap meaning special in the sense documented by
vm_normal_page(). The implication here would be that if you register
memory like above, TTM would never be able to set up a huge page table
entry to it. But it sounds like that's not an issue?
/Thomas
/Thomas
-Daniel
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