On Wed, 22 Sep 2021, Peter Xu wrote: > > Not installing pmd means uffd-minor can still trap any further faults just like > before, afaiu. > > There's a very trivial detail that the pmd missing case will have a very slight > code path change when the next page fault happens: in __handle_mm_fault() we'll > first try to go into create_huge_pmd() once, however since shmem didn't provide > huge_fault(), we'll go the VM_FAULT_FALLBACK path, and things will go like > before when faulting on a small pte. The next UFFDIO_CONTINUE will allocate > that missing pmd again, however it'll install a 4K page only. I think you're mistaken there. I can't tell you much about ->huge_fault(), something introduced for DAX I believe; but shmem has managed pmd mappings without it, since before ->huge_fault() was ever added. Look for the call to do_set_pmd() in finish_fault(): I think you'll find that is the way shmem's huge pmds get in. Earlier in the thread you suggested "shmem_getpage() only returns small pages": but it can very well return PageTransCompound pages, head or tail, which arrive at this do_set_pmd(). Hugh
