On Tue, Oct 29, 2024 at 03:16:00PM +0000, Lorenzo Stoakes wrote: > On Tue, Oct 29, 2024 at 03:04:41PM +0000, Catalin Marinas wrote: > > On Mon, Oct 28, 2024 at 10:14:50PM +0000, Lorenzo Stoakes wrote: > > > So continue to check VM_MTE_ALLOWED which arch_calc_vm_flag_bits() sets if > > > MAP_ANON. > > [...] > > > diff --git a/mm/shmem.c b/mm/shmem.c > > > index 4ba1d00fabda..e87f5d6799a7 100644 > > > --- a/mm/shmem.c > > > +++ b/mm/shmem.c > > > @@ -2733,9 +2733,6 @@ static int shmem_mmap(struct file *file, struct vm_area_struct *vma) > > > if (ret) > > > return ret; > > > > > > - /* arm64 - allow memory tagging on RAM-based files */ > > > - vm_flags_set(vma, VM_MTE_ALLOWED); > > > > This breaks arm64 KVM if the VMM uses shared mappings for the memory > > slots (which is possible). We have kvm_vma_mte_allowed() that checks for > > the VM_MTE_ALLOWED flag as the VMM may not use PROT_MTE/VM_MTE directly. > > Ugh yup missed that thanks. > > > I need to read this thread properly but why not pass the file argument > > to arch_calc_vm_flag_bits() and set VM_MTE_ALLOWED in there? > > Can't really do that as it is entangled in a bunch of other stuff, > e.g. calc_vm_prot_bits() would have to pass file and that's used in a bunch > of places including arch code and... etc. etc. Not calc_vm_prot_bits() but calc_vm_flag_bits(). arch_calc_vm_flag_bits() is only implemented by two architectures - arm64 and parisc and calc_vm_flag_bits() is only called from do_mmap(). Basically we want to set VM_MTE_ALLOWED early during the mmap() call and, at the time, my thinking was to do it in calc_vm_flag_bits(). The calc_vm_prot_bits() OTOH is also called on the mprotect() path and is responsible for translating PROT_MTE into a VM_MTE flag without any checks. arch_validate_flags() would check if VM_MTE comes together with VM_MTE_ALLOWED. But, as in the KVM case, that's not the only function checking VM_MTE_ALLOWED. Since calc_vm_flag_bits() did not take a file argument, the lazy approach was to add the flag explicitly for shmem (and hugetlbfs in -next). But I think it would be easier to just add the file argument to calc_vm_flag_bits() and do the check in the arch code to return VM_MTE_ALLOWED. AFAICT, this is called before mmap_region() and arch_validate_flags() (unless I missed something in the recent reworking). > I suggest instead we instead don't drop the yucky shmem thing, which will > set VM_MTE_ALLOWED for shmem, with arch_calc_vm_flag_bits() still setting > it for MAP_ANON, but the other changes will mean the arch_validate_flags() > will be fixed too. > > So this just means not dropping the mm/shmem.c bit basically and everything > should 'just work'? If we can't get the calc_vm_flag_bits() approach to work, I'm fine with this as a fix and we'll look to do it properly from 6.13. -- Catalin
