* 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
