On 5/13/26 10:51, Thomas Hellström wrote: > On Wed, 2026-05-13 at 10:37 +0200, David Hildenbrand (Arm) wrote: >> On 5/13/26 09:47, Christian König wrote: >>> Hi David & Thomas, >>> >>> ... >>> >>> Exactly that is one of the major reasons why we aren't using a >>> shmem as backing store for TTM buffers in the first place. >> >> What was the problem with that the last time this was considered? >> >> shmem nowadays supports THP (e.g., 2M) and even mTHP (e.g., 64K). >> >> For internal mounts, it must be enabled accordingly >> (/sys/kernel/mm/transparent_hugepage/.../shmem_enabled). >> >> Some distributions still default to "never". I guess if an admin >> enables it, you >> would just get THPs. > > FWIW, the i915 driver which uses shmem "natively" uses a special mount > here that gives back THPs. > >> >> If "distro default" is the only problem, I guess we could think about >> how to >> improve that. For example, just let internal GPU DRM objects allocate >> any folio >> size available and supported etc. >> >> Would that make it possible to just use shmem natively? (e.g., how >> would this >> interact with shmem features like folio migration, would that be >> workable with >> DRM objects?). > > Currently the drivers that use shmem in this way use > "mapping_set_unevictable()" as long as the object is bound to the GPU. > Then shrinkers can unbind from GPU and revert that setting.
Right, but mapping_set_unevictable() only affects folio_evictable() - -reclaim behavior. Not other properties (such as folio migration). > > The problem, (as also stated in the cover letter of this series) is for > drivers that need to change caching of the pages to WC or UC. I assume you mean "To be able to easily maintain pools of pages mapped uncached or write-combined". Can you point me at the code that changes the caching of the pages? > That's an > extremely costly operation so TTM needs to pool such allocations. > That's where using shmem natively becomes very ugly, because you can't > really use a 1:1 mapping between shmem objects and DRM objects anymore. So you would require different caching attributes within a DRM object? -- Cheers, David
