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?
