Re: [PATCH hotfix 6.12 v4 4/5] mm: refactor arch_calc_vm_flag_bits() and arm64 MTE handling

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Oct 30, 2024 at 10:18:27AM +0100, Vlastimil Babka wrote:
> On 10/29/24 19:11, Lorenzo Stoakes wrote:
> > --- a/arch/arm64/include/asm/mman.h
> > +++ b/arch/arm64/include/asm/mman.h
> > @@ -6,6 +6,8 @@
> > 
> >  #ifndef BUILD_VDSO
> >  #include <linux/compiler.h>
> > +#include <linux/fs.h>
> > +#include <linux/shmem_fs.h>
> >  #include <linux/types.h>
> > 
> >  static inline unsigned long arch_calc_vm_prot_bits(unsigned long prot,
> > @@ -31,19 +33,21 @@ static inline unsigned long arch_calc_vm_prot_bits(unsigned long prot,
> >  }
> >  #define arch_calc_vm_prot_bits(prot, pkey) arch_calc_vm_prot_bits(prot, pkey)
> > 
> > -static inline unsigned long arch_calc_vm_flag_bits(unsigned long flags)
> > +static inline unsigned long arch_calc_vm_flag_bits(struct file *file,
> > +						   unsigned long flags)
> >  {
> >  	/*
> >  	 * Only allow MTE on anonymous mappings as these are guaranteed to be
> >  	 * backed by tags-capable memory. The vm_flags may be overridden by a
> >  	 * filesystem supporting MTE (RAM-based).
> 
> We should also eventually remove the last sentence or even replace it with
> its negation, or somebody might try reintroducing the pattern that won't
> work anymore (wasn't there such a hugetlbfs thing in -next?).

I agree, we should update this comment as well though as a fix this
patch is fine for now.

There is indeed a hugetlbfs change in -next adding VM_MTE_ALLOWED. It
should still work after the above change but we'd need to move it over
here (and fix the comment at the same time). We'll probably do it around
-rc1 or maybe earlier once this fix hits mainline. I don't think we have
an equivalent of shmem_file() for hugetlbfs, we'll need to figure
something out.

> >  	 */
> > -	if (system_supports_mte() && (flags & MAP_ANONYMOUS))
> > +	if (system_supports_mte() &&
> > +	    ((flags & MAP_ANONYMOUS) || shmem_file(file)))
> >  		return VM_MTE_ALLOWED;
> > 
> >  	return 0;
> >  }

This will conflict with the arm64 for-next/core tree as it's adding
a MAP_HUGETLB check. Trivial resolution though, Stephen will handle it.

-- 
Catalin