Re: [PATCH v1] x86/mm/pat: fix VM_PAT handling in COW mappings

[Ingo Molnar]

* David Hildenbrand <david@xxxxxxxxxx> wrote:

> On 26.03.24 09:33, Ingo Molnar wrote:
> > 
> > * David Hildenbrand <david@xxxxxxxxxx> wrote:
> > 
> > > On 12.03.24 20:22, Matthew Wilcox wrote:
> > > > On Tue, Mar 12, 2024 at 07:11:18PM +0100, David Hildenbrand wrote:
> > > > > PAT handling won't do the right thing in COW mappings: the first PTE
> > > > > (or, in fact, all PTEs) can be replaced during write faults to point at
> > > > > anon folios. Reliably recovering the correct PFN and cachemode using
> > > > > follow_phys() from PTEs will not work in COW mappings.
> > > > 
> > > > I guess the first question is: Why do we want to support COW mappings
> > > > of VM_PAT areas?  What breaks if we just disallow it?
> > > 
> > > Well, that was my first approach. Then I decided to be less radical (IOW
> > > make my life easier by breaking less user space) and "fix it" with
> > > minimal effort.
> > > 
> > > Chances of breaking some weird user space is possible, although I believe
> > > for most such mappings MAP_PRIVATE doesn't make too much sense sense.
> > > 
> > > Nasty COW support for VM_PFNMAP mappings dates back forever. So does PAT
> > > support.
> > > 
> > > I can try finding digging through some possible user space users
> > > tomorrow.
> > 
> > I'd much prefer restricting VM_PAT areas than expanding support. Could we
> 
> Note that we're not expanding support, we're fixing what used to be
> possible before but mostly broke silently.

Yeah - that's de-facto expanding support. :-)

> But I agree that we should rather remove these corner cases instead of 
> fixing them.

Yeah, especially if no code is hitting it intentionally.

> > try the trivial restriction approach first, and only go with your original
> > patch if that fails?
> 
> Which version would you prefer, I had two alternatives (excluding comment
> changes, white-space expected to be broken).
> 
> 
> 1) Disallow when we would have set VM_PAT on is_cow_mapping()
> 
> diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
> index 0d72183b5dd0..6979912b1a5d 100644
> --- a/arch/x86/mm/pat/memtype.c
> +++ b/arch/x86/mm/pat/memtype.c
> @@ -994,6 +994,9 @@ int track_pfn_remap(struct vm_area_struct *vma, pgprot_t *prot,
>                                 && size == (vma->vm_end - vma->vm_start))) {
>                 int ret;
> +               if (is_cow_mapping(vma->vm_flags))
> +                       return -EINVAL;
> +
>                 ret = reserve_pfn_range(paddr, size, prot, 0);
>                 if (ret == 0 && vma)
>                         vm_flags_set(vma, VM_PAT);
> 
> 
> 2) Fallback to !VM_PAT
> 
> diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
> index 0d72183b5dd0..8e97156c9be8 100644
> --- a/arch/x86/mm/pat/memtype.c
> +++ b/arch/x86/mm/pat/memtype.c
> @@ -990,8 +990,8 @@ int track_pfn_remap(struct vm_area_struct *vma, pgprot_t *prot,
>         enum page_cache_mode pcm;
>         /* reserve the whole chunk starting from paddr */
> -       if (!vma || (addr == vma->vm_start
> -                               && size == (vma->vm_end - vma->vm_start))) {
> +       if (!vma || (!is_cow_mapping(vma->vm_flags) && addr == vma->vm_start &&
> +                    size == (vma->vm_end - vma->vm_start))) {
>                 int ret;
>                 ret = reserve_pfn_range(paddr, size, prot, 0);
>
> 
> 
> Personally, I'd go for 2).

So what's the advantage of #2? This is clearly something the user didn't 
really intend or think about much. Isn't explicitly failing that mapping a 
better option than silently downgrading it to !VM_PAT?

(If I'm reading it right ...)

Thanks,

	Ingo