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Date: Sat, 29 Aug 2026 14:00:03 +0300
From: Mike Rapoport <rppt@kernel.org>
To: "David Hildenbrand (Arm)" <david@kernel.org>
Cc: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Baolin Wang <baolin.wang@linux.alibaba.com>,
	Barry Song <baohua@kernel.org>, Dev Jain <dev.jain@arm.com>,
	Hugh Dickins <hughd@google.com>, Jann Horn <jannh@google.com>,
	Jason Gunthorpe <jgg@ziepe.ca>, John Hubbard <jhubbard@nvidia.com>,
	Jonathan Corbet <corbet@lwn.net>, Lance Yang <lance.yang@linux.dev>,
	"Liam R. Howlett" <liam@infradead.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Michal Hocko <mhocko@suse.com>, Muchun Song <muchun.song@linux.dev>,
	Nico Pache <nico.pache@linux.dev>,
	Oscar Salvador <osalvador@suse.de>,
	Pedro Falcato <pfalcato@suse.de>, Peter Xu <peterx@redhat.com>,
	Ryan Roberts <ryan.roberts@arm.com>,
	Shakeel Butt <shakeel.butt@linux.dev>,
	Shuah Khan <skhan@linuxfoundation.org>,
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	Suren Baghdasaryan <surenb@google.com>,
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	linux-trace-kernel@vger.kernel.org
Subject: Re: [PATCH 6/6] userfaultfd: collapse VM_UFFD_{MISSING,WP,MINOR,RWP}
 into single VM_UFFD
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References: <20260823-uffd-vm-flags-v1-v1-0-3086981b33cf@kernel.org>
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In-Reply-To: <37f6af95-950e-4457-886e-6548d4719f0d@kernel.org>

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.