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 11:53:06AM +0000, Lorenzo Stoakes wrote:
> On Wed, Oct 30, 2024 at 12:09:43PM +0100, Vlastimil Babka wrote:
> > On 10/30/24 11:58, Catalin Marinas wrote:
> > > 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
> >
> > I guess it will work after the above change, but not after 5/5?
> >
> > > 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 assume this will hopefully go to rc7.
> 
> To be clear - this is a CRITICAL fix that MUST land for 6.12. I'd be inclined to
> try to get it to an earlier rc-.

Ah, good point. So after this series is merged at rc6/rc7, the new
MTE+hugetlbfs in -next won't work. Not an issue, it can be sorted out
later.

> > > I don't think we have
> > > an equivalent of shmem_file() for hugetlbfs, we'll need to figure
> > > something out.
> >
> > I've found is_file_hugepages(), could work? And while adding the hugetlbfs
> > change here, the comment could be adjusted too, right?
> 
> Right but the MAP_HUGETLB should work to? Can we save such changes that
> alter any kind of existing behaviour to later series?
> 
> As this is going to be backported (by me...!) and I don't want to risk
> inadvertant changes.

MAP_HUGETLB and is_file_hugepages() fixes can go in after 6.13-rc1. This
series is fine as is, we wouldn't backport any MAP_HUGETLB changes
anyway since the flag check wasn't the only issue that needed addressing
for hugetlb MTE mappings.

-- 
Catalin