On Tue, Jun 24, 2025 at 04:37:26PM -0400, Peter Xu wrote: > On Thu, Jun 19, 2025 at 03:40:41PM -0300, Jason Gunthorpe wrote: > > Even with this new version you have to decide to return PUD_SIZE or > > bar_size in pci and your same reasoning that PUD_SIZE make sense > > applies (though I would probably return bar_size and just let the core > > code cap it to PUD_SIZE) > > Yes. > > Today I went back to look at this, I was trying to introduce this for > file_operations: > > int (*get_mapping_order)(struct file *, unsigned long, size_t); > > It looks almost good, except that it so far has no way to return the > physical address for further calculation on the alignment. > > For THP, VA is always calculated against pgoff not physical address on the > alignment. I think it's OK for THP, because every 2M THP folio will be > naturally 2M aligned on the physical address, so it fits when e.g. pgoff=0 > in the calculation of thp_get_unmapped_area_vmflags(). > > Logically it should even also work for vfio-pci, as long as VFIO keeps > using the lower 40 bits of the device_fd to represent the bar offset, > meanwhile it'll also require PCIe spec asking the PCI bars to be mapped > aligned with bar sizes. > > But from an API POV, get_mapping_order() logically should return something > for further calculation of the alignment to get the VA. pgoff here may not > always be the right thing to use to align to the VA: after all, pgtable > mapping is about VA -> PA, the only reasonable and reliable way is to align > VA to the PA to be mappped, and as an API we shouldn't assume pgoff is > always aligned to PA address space. My feeling, and the reason I used the phrase "pgoff aligned address", is that the owner of the file should already ensure that for the large PTEs/folios: pgoff % 2**order == 0 physical % 2**order == 0 So, things like VFIO do need to hand out high alignment pgoffs to make this work - which it already does. To me this just keeps thing simpler. I guess if someone comes up with a case where they really can't get a pgoff alignment and really need a high order mapping then maybe we can add a new return field of some kind (pgoff adjustment?) but that is so weird I'd leave it to the future person to come and justfiy it. Jason
