Where do ttm->pages come from?

[Matthew Wilcox]

On Tue, Mar 18, 2025 at 09:37:42AM +0100, Christian König wrote:
> Hi Matthew,
> 
> you actually got your understanding exactly inverted :)

Thanks; that's very helpful!

> Amdgpu takes a look at page->mapping to make sure they are *not* coming from the call sequence starting with amdgpu_ttm_tt_set_user_pages() or be imported from another driver through DMA-buf etc...
> 
> The only pages which are allowed to pass this check are the private driver allocated ones, see this code function amdgpu_ttm_tt_populate():
> 
>         for (i = 0; i < ttm->num_pages; ++i)
>                 ttm->pages[i]->mapping = bdev->dev_mapping;
> 
> So far that has been save since nobody else should be using the address space object from our drivers inode under /dev.
> 
> But when you plan to remove page->mapping we probably have a problem here.

OK.  This makes the problem a lot more tractable.

>From the MM side, we have a short-term and a long-term goal.  The
long-term goal is to reduce struct page to a single tagged pointer
called a memdesc.  Details here:

https://kernelnewbies.org/MatthewWilcox/Memdescs

In the short term, we're removing page->mapping, ->index, ->lru and
->memcg_data, leaving ->flags, ->_refcount and ->private as well as the
newly-introduced ->memdesc.

So if all you needed to do was identify which pages belong to you,
I'd suggest setting ->private to a unique value (eg bdev->dev_mapping).
But it feels like you need more than that; you need a refcount, and you
need to map pages to userspace.  Do you need to support GUP on these
pages (ie can it be the user buffer for O_DIRECT, can it be registered
with RDMA, that kind of thing), or is this memory just for userspace to
do load/store, and no fancy stuff is allowed?

At this point, I'd suggest that you allocate folios rather than pages.
That lets us continue to use folio->mapping to distinguish these pages.
There would be an opportunity to save some memory in the future by using
a slimmer control structure once we understand the new system better,
but maybe it's OK to just keep using a folio forever.

Thoughts?