Re: [PATCH] drm/amdkfd: Fix svm_bo and vram page refcount

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Oct 03, 2025 at 06:16:14PM -0400, Felix Kuehling wrote:

> It sounds like zone_device_page_init is just unsafe to use in
> general. 

It can only be used if you know the page is freed.

> It assumes that pages have a 0 refcount. 

Yes

> But I don't see a good way for drivers to guarantee that, because
> they are not in control of when the page refcounts for their
> zone-device pages get decremented.

?? Drivers are supposed to hoook pgmap->ops->page_free() and keep
track.

There is no way to write a driver without calling
zone_device_page_init() as there is no other defined way to re-use a
page that has been returned through page_free().

It is completely wrong to call get_page() on a 0 refcount folio, we
don't have a debugging crash for this, but we really should. If you
think the refcount could be 0 you have to use a try_get().

So this patch looks wrong to me, I see a page_free() implementation
and this is the only call to zone_device_page_init(). If you remove it
the driver is absolutely broken.

I would expect migration should be writing to freed memory and
zone_device_page_init() is the correct and only way to make freed
memory usable again.

Therefore, I expect the refcount to be 0 when
svm_migrate_ram_to_vram() picks a dst.

If it is not true, and you are tring to migrate to already allocated
VRAM, then WTF?

And if you really want to do that then yes you need to use get_page
but you need a different path to handle already allocated vs
page_free() called. get_page() MUST NOT be used to unfree page_free'd
memory.

The explanation in the commit doesn't really have enough detail:

> 1. CPU page fault handler get vram page, migrate the vram page to
>    system page
> 2. GPU page fault migrate to the vram page, set page refcount to 1

So why is the same vram page being used for both? For #1 the VRAM page
is installed in a swap entry so it is has an elevated refcount.

The implication is that #2 is targeting already allocated VRAM memory
that is NOT FREE.

Jason