On Tue, Jun 17, 2025 at 07:36:08PM -0400, Peter Xu wrote: > On Tue, Jun 17, 2025 at 08:18:07PM -0300, Jason Gunthorpe wrote: > > On Tue, Jun 17, 2025 at 04:56:13PM -0400, Peter Xu wrote: > > > On Mon, Jun 16, 2025 at 08:00:11PM -0300, Jason Gunthorpe wrote: > > > > On Mon, Jun 16, 2025 at 06:06:23PM -0400, Peter Xu wrote: > > > > > > > > > Can I understand it as a suggestion to pass in a bitmask into the core mm > > > > > API (e.g. keep the name of mm_get_unmapped_area_aligned()), instead of a > > > > > constant "align", so that core mm would try to allocate from the largest > > > > > size to smaller until it finds some working VA to use? > > > > > > > > I don't think you need a bitmask. > > > > > > > > Split the concerns, the caller knows what is inside it's FD. It only > > > > needs to provide the highest pgoff aligned folio/pfn within the FD. > > > > > > Ultimately I even dropped this hint. I found that it's not really > > > get_unmapped_area()'s job to detect over-sized pgoffs. It's mmap()'s job. > > > So I decided to avoid this parameter as of now. > > > > Well, the point of the pgoff is only what you said earlier, to adjust > > the starting alignment so the pgoff aligned high order folios/pfns > > line up properly. > > I meant "highest pgoff" that I dropped. > > We definitely need the pgoff to make it work. So here I dropped "highest > pgoff" passed from the caller because I decided to leave such check to the > mmap() hook later. > > > > > > > The mm knows what leaf page tables options exist. It should try to > > > > align to the closest leaf page table size that is <= the FD's max > > > > aligned folio. > > > > > > So again IMHO this is also not per-FD information, but needs to be passed > > > over from the driver for each call. > > > > It is per-FD in the sense that each FD is unique and each range of > > pgoff could have a unique maximum. > > > > > Likely the "order" parameter appeared in other discussions to imply a > > > maximum supported size from the driver side (or, for a folio, but that is > > > definitely another user after this series can land). > > > > Yes, it is the only information the driver can actually provide and > > comes directly from what it will install in the VMA. > > > > > So far I didn't yet add the "order", because currently VFIO definitely > > > supports all max orders the system supports. Maybe we can add the order > > > when there's a real need, but maybe it won't happen in the near > > > future? > > > > The purpose of the order is to prevent over alignment and waste of > > VMA. Your technique to use the length to limit alignment instead is > > good enough for VFIO but not very general. > > Yes that's also something I didn't like. I think I'll just go ahead and > add the order parameter, then use it in previous patch too. So I changed my mind, slightly. I can still have the "order" parameter to make the API cleaner (even if it'll be a pure overhead.. because all existing caller will pass in PUD_SIZE as of now), but I think I'll still stick with the ifdef in patch 4, as I mentioned here: https://lore.kernel.org/all/aFGMG3763eSv9l8b@x1.local/ The problem is I just noticed yet again that exporting huge_mapping_get_va_aligned() for all configs doesn't make sense. At least it'll need something like this to make !MMU compile for VFIO, while this is definitely some ugliness I also want to avoid.. ===8<=== diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h index 59fdafb1034b..f40a8fb64eaa 100644 --- a/include/linux/huge_mm.h +++ b/include/linux/huge_mm.h @@ -548,7 +548,11 @@ static inline unsigned long huge_mapping_get_va_aligned(struct file *filp, unsigned long addr, unsigned long len, unsigned long pgoff, unsigned long flags) { +#ifdef CONFIG_MMU return mm_get_unmapped_area(current->mm, filp, addr, len, pgoff, flags); +#else + return 0; +#endif } static inline bool ===8<=== The issue is still mm_get_unmapped_area() is only exported on CONFIG_MMU, so we need to special case that for huge_mapping_get_va_aligned(), and here for !THP && !MMU. Besides the ugliness, it's also about how to choose a default value to return when mm_get_unmapped_area() isn't available. I gave it a defalut value (0) as example, but I don't even thnk that 0 makes sense. It would (if ever triggerable from any caller on !MMU) mean it will return 0 directly to __get_unmapped_area() and further do_mmap() (of !MMU code, which will come down from ksys_mmap_pgoff() of nommu.c) will take that addr=0 to be the addr to mmap.. that sounds wrong. There's just no way to provide a sane default value for !MMU. So going one step back: huge_mapping_get_va_aligned() (or whatever name we prefer) doesn't make sense to be exported always, but only when CONFIG_MMU. It should follow the same way we treat mm_get_unmapped_area(). Here it also goes back to the question on why !MMU even support mmap(): https://www.kernel.org/doc/Documentation/nommu-mmap.txt So, for the case of v4l driver (v4l2_m2m_get_unmapped_area that I used to quote, which only defines in !MMU and I used to misread..), for example, it's really a minimal mmap() support on ucLinux and that's all about that. My gut feeling is the noMMU use case more or less abused the current get_unmapped_area() hook to provide the physical addresses, so as to make mmap() work even on ucLinux. It's for sure not a proof that we should have huge_mapping_get_va_aligned() or mm_get_unmapped_area() availalbe even for !MMU. That's all about VAs and that do not exist in !MMU as a concept. Thanks, -- Peter Xu
