Re: [PATCH 6/6] userfaultfd: collapse VM_UFFD_{MISSING,WP,MINOR,RWP} into single VM_UFFD

[Mike Rapoport]

On Thu, Aug 27, 2026 at 05:19:30PM +0200, David Hildenbrand (Arm) wrote:
> On 8/27/26 13:18, Lorenzo Stoakes (ARM) wrote:
> > On Thu, Aug 27, 2026 at 12:16:32PM +0100, Lorenzo Stoakes (ARM) wrote:
> >> So actually you're increasing by a cacheline and increasing the VMA size by
> >> 64 bytes, i.e. 1/3, which is unacceptable obviously.
> >>
> >> Maybe there's something that can be done with:
> >>
> >> 		/* forced alignments: 1 */
> >>
> >> Perhaps? But that looks potentially ugly.
> >>
> > 
> > I say elsewhere (or think I do) but to highlight - I think probably we could fix
> > this by putting the flags in the low bits of vm_uffd_state.ctx?
> 
> if that's possible that would be clearly preferable memory-wise.

This gives only 4 bits and makes this completely not extendable.

So I think I'll drop this for now and wait until VMA grows another cache
line or until having per-VMA uffd state rather than a pointer to per-fd
context is a must.
 
> -- 
> Cheers,
> 
> David

-- 
Sincerely yours,
Mike.