Re: + fix-zero-copy-i-o-on-__get_user_pages-allocated-pages.patch added to mm-hotfixes-unstable branch

[Peter Xu <peterx@redhat.com>]

On Thu, May 08, 2025 at 05:36:12PM +0300, Pantelis Antoniou wrote:
> On Thu, 8 May 2025 10:16:31 -0400
> Peter Xu <peterx@redhat.com> wrote:
> 
> Hi Peter,

Hi, Pantelis,

[...]

> > > @@ -1271,8 +1287,6 @@ static int check_vma_flags(struct vm_are
> > >  	int foreign = (gup_flags & FOLL_REMOTE);
> > >  	bool vma_anon = vma_is_anonymous(vma);
> > >  
> > > -	if (vm_flags & (VM_IO | VM_PFNMAP))
> > > -		return -EFAULT;
> > 
> > Is there's any justification that this won't break some existing GUP
> > users that may rely on properly failing at pfnmaps?
> > 
> > IIUC netfs isn't the first one that wants to GUP on top of pfnmaps,
> > KVM does it for years and so far it was processed in a standalone
> > path:
> > 
> > hva_to_pfn:
> > 	else if (vma->vm_flags & (VM_IO | VM_PFNMAP)) {
> > 		r = hva_to_pfn_remapped(vma, kfp, &pfn);
> > 
> > That started with supporting real pfnmaps (with no page struct), but
> > pfnmap with page structs can also happen afaict, and kvm processes
> > that too by checking page==NULL ultimately, e.g. in
> > kvm_release_faultin_page().
> > 
> 
> I see. The problem is that we're not the owners of the code in netfslib,
> and it is considerably more intrusive to fix things there.
> 
> This is a hotfix for a userspace regression. I sort of agree that having
> different handling for these areas in netfslib would be ideal.

Do you mean this used to work in older kernels?  Some more info on the
regression would be more than welcomed if so..  If it fixes a kernel
regression, we may want a Fixes for whatever patch at last.

Or do you mean it's a regression caused by userspace change?

Thanks,

-- 
Peter Xu