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

[Tal Zussman <tz2294@columbia.edu>]

On 2026-08-29 14:00 +0300, Mike Rapoport wrote:
> 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.

Wouldn't it be 6 bits? struct userfaultfd_ctx is allocated with
kmem_cache_create() and SLAB_HWCACHE_ALIGN, and most of the flags are
only available on 64 bit, so it should be 64-byte aligned in the
relevant cases.

(Not that 6 is that much better than 4... but it's a little more wiggle
room.)

It could in theory also be bumped up to 7 by setting align in
kmem_cache_create(). userfaultfd_ctx already takes 192 bytes due to
existing alignment. Aligning it to 128 bytes would make it 256 bytes,
adding 64 bytes to each uffd rather than each VMA. But this sounds like
more pain for little gain :)

> 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.
> 
>