Re: [PATCH] mm: Add new vma flag VM_LOCAL_CPU

[Matthew Wilcox]

On Mon, May 14, 2018 at 10:37:38PM +0300, Boaz Harrosh wrote:
> On 14/05/18 22:15, Matthew Wilcox wrote:
> > On Mon, May 14, 2018 at 08:28:01PM +0300, Boaz Harrosh wrote:
> >> On a call to mmap an mmap provider (like an FS) can put
> >> this flag on vma->vm_flags.
> >>
> >> The VM_LOCAL_CPU flag tells the Kernel that the vma will be used
> >> from a single-core only, and therefore invalidation (flush_tlb) of
> >> PTE(s) need not be a wide CPU scheduling.
> > 
> > I still don't get this.  You're opening the kernel up to being exploited
> > by any application which can persuade it to set this flag on a VMA.
> > 
> 
> No No this is not an application accessible flag this can only be set
> by the mmap implementor at ->mmap() time (Say same as VM_VM_MIXEDMAP).
> 
> Please see the zuf patches for usage (Again apologise for pushing before
> a user)
> 
> The mmap provider has all the facilities to know that this can not be
> abused, not even by a trusted Server.

I don't think page tables work the way you think they work.

+               err = vm_insert_pfn_prot(zt->vma, zt_addr, pfn, prot);

That doesn't just insert it into the local CPU's page table.  Any CPU
which directly accesses or even prefetches that address will also get
the translation into its cache.