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

[Mike Rapoport]

On Sat, Aug 29, 2026 at 02:42:52PM -0400, Tal Zussman wrote:
> 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.)

I did remember that SLAB_HWCACHE_ALIGN could be as small as 16 bytes, but I
didn't verify it for architectures that support fancy uffd modes.
 
> 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 :)

Yeah, even with as plenty as 7 bits :)
 
> > 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.
> > 
> > 
> 
> 

-- 
Sincerely yours,
Mike.