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

[Ingo Molnar]

* 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 
try the trivial restriction approach first, and only go with your original 
patch if that fails?

Thanks,

	Ingo