Re: [PATCH] mm/x86/pat: Only untrack the pfn range if unmap region

Jason Gunthorpe
2024-07-23T21:44:21Z

On Tue, Jul 23, 2024 at 11:36:51PM +0200, David Hildenbrand wrote:
> I wonder if we could just let relevant users do the PAT handling manually:
> I'm also not sure how many remap_pfn_range users end up triggering VM_PAT
> code although they don't really have to (just because they happen to cover a
> full VMA)?
>
> One nasty thing is fork(), I was wondering if relevant users really rely on
> that or if we could force these VMAs to simply not get copied during fork.
> During fork() we have to "duplicate" the reservation ...

I admit I barely understand what x86 uses this PAT stuff for -
allowing WC mappings is part of it?

If yes, then RDMA would expect WC PFN MAP VMAs to copy their WCness
during fork..

Jason