On Tue, Feb 04, 2025 at 03:38:17PM +0100, David Hildenbrand wrote: > > > It can still race with memory offlining, and it refuses ZONE_DEVICE > > > pages. For the latter, we have a different way to check validity. See > > > memory_failure() that first calls pfn_to_online_page() to then check > > > get_dev_pagemap(). > > > > I'll give it a shot with these functions. If they work for my use case, > > then it's good to have extra checks and I'll add them for v2. Thanks! > > Let me know if you run into any issues. > > > > > > > > > > > > > > If the answer is "no" then that's fine. It's still an unsafe function > > > > and we need to document in the safety section that it should only be > > > > used for memory that is either known to be allocated and pinned and will > > > > not be freed while the `struct page` is borrowed, or memory that is > > > > reserved and not owned by the buddy allocator, so in practice correct > > > > use would not be racy with memory hot-remove anyway. > > > > > > > > This is already the case for the drm/asahi use case, where the pfns > > > > looked up will only ever be one of: > > > > > > > > - GEM objects that are mapped to the GPU and whose physical pages are > > > > therefore pinned (and the VM is locked while this happens so the objects > > > > cannot become unpinned out from under the running code), > > > > > > How exactly are these pages pinned/obtained? > > > > Under the hood it's shmem. For pinning, it winds up at > > `drm_gem_get_pages()`, which I think does a `shmem_read_folio_gfp()` on > > a mapping set as unevictable. > > Thanks. So we grab another folio reference via > shmem_read_folio_gfp()->shmem_get_folio_gfp(). > > Hm, I wonder if we might end up holding folios residing in > ZONE_MOVABLE/MIGRATE_CMA longer than we should. > > Compared to memfd_pin_folios(), which simulates FOLL_LONGTERM and makes sure > to migrate pages out of ZONE_MOVABLE/MIGRATE_CMA. > > But that's a different discussion, just pointing it out, maybe I'm missing > something :) Good GPU Drivers (tm) are supposed to have a shrinker so we can at least nuke some of them again. Some folks even looked into hooking up a migrate callback through the address_space (or wherever that hook was, this is from memory) so we can make this somewhat reliable. So yeah we're hogging ZONE_MOVEABLE unduly still. The other side is that there's about 2-3 good drivers (msm, i915, xe should have a shrinker now too but I didn't check). The others all fall various levels of short, or still have 3 times cargo-culted versions of i915's pin-as-a-lock design and get it completely wrong. So yeah I'm aware this isn't great, and we're at least glacially slowly moving towards a common shrinker infrastructure that maybe in a glorious future gets all this right. I mean it took us 15+ years to get to a cgroups controller after all too, and that was also a well known issue of just being able to hog memory with no controls and potentially cause havoc. Cheers, Sima -- Simona Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
