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 7wiv4dOy 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/13/26 10:37, David Hildenbrand (Arm) wrote: > On 5/13/26 09:47, Christian König wrote: >> Hi David & Thomas, >> >> On 5/12/26 22:03, David Hildenbrand (Arm) wrote: >>> On 5/12/26 13:31, Thomas Hellström wrote: >> ... >>>> >>>> OK, can eliminate those. Is VM_WARN_ON_FOLIO() preferred, >>>> or any other type of assert? >>> >>> VM_WARN_ON_FOLIO() is usually what you want, or VM_WARN_ON_ONCE(). >>> >>>> >>>> >>>> OK, let me understand the concern. The pages are allocated as multi- >>>> page folios using alloc_pages(gfp, order), but typically not promoted >>>> to compound pages, until inserted here. Is it that promotion that is of >>>> concern or inserting pages of unknown origin into shmem? Anything we >>>> can do to alleviate that concern? >>> >>> It's all rather questionable. >>> >>> A couple of points: >>> >>> a) The pages are allocated to be unmovable, but adding them to shmem effectively >>> turns them movable. Now you interfere with the page allocator logic of >>> placing movable and unmovable pages a reasonable way into >>> pageblocks that group allocations of similar types. >>> >>> b) A driver is not supposed to decide which folio size will be allocated for >>> shmem. >> >> 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. Yeah, exactly that is not acceptable. We have some customers who already use that approach through udmabuf, so we already have some experience with it. And I can't count how often I had to explain that it's a configuration issue and that the admin has to enable THP to get decent performance. > 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. Mhm, that sounds not so bad. I think what drivers really need is that they can give the order to shmem_read_folio_gfp() and get a folio with that order or -ENOMEM as return. In other words we need to enforce it and if the desired page size doesn't work we can then still decide if we want a fallback or not based on the use case the driver tries to implement. > 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?). Mostly, I mean there is still the use case for UC and USWC memory but at least for AMD GPUs that is mostly negligible (we need it for a handfull of workarounds for HW bugs etc...). Thanks, Christian.