linux-kernel - Re: [PATCH v2 3/5] mm: introduce VM_FAULT_UFFD

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aSgzcpFP1qBda5ef@kernel.org>
Date: Thu, 27 Nov 2025 13:18:10 +0200
From: Mike Rapoport <rppt@...nel.org>
To: Peter Xu <peterx@...hat.com>
Cc: linux-mm@...ck.org, Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	David Hildenbrand <david@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	James Houghton <jthoughton@...gle.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Michal Hocko <mhocko@...e.com>,
	Nikita Kalyazin <kalyazin@...zon.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Sean Christopherson <seanjc@...gle.com>,
	Shuah Khan <shuah@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, linux-kselftest@...r.kernel.org,
	"David Hildenbrand (Red Hat)" <david@...nel.org>
Subject: Re: [PATCH v2 3/5] mm: introduce VM_FAULT_UFFD_MINOR fault reason

On Tue, Nov 25, 2025 at 02:21:16PM -0500, Peter Xu wrote:
> Hi, Mike,
> 
> On Tue, Nov 25, 2025 at 08:38:38PM +0200, Mike Rapoport wrote:
> > From: "Mike Rapoport (Microsoft)" <rppt@...nel.org>
> > 
> > When a VMA is registered with userfaulfd in minor mode, its ->fault()
> > method should check if a folio exists in the page cache and if yes
> > ->fault() should call handle_userfault(VM_UFFD_MISSING).
> 
> s/MISSING/MINOR/

Thanks, fixed. 

> > new VM_FAULT_UFFD_MINOR there instead.
> 
> Personally I'd keep the fault path as simple as possible, because that's
> the more frequently used path (rather than when userfaultfd is armed). I
> also see it slightly a pity that even with flags introduced, it only solves
> the MINOR problem, not MISSING.

With David's suggestion the likely path remains unchanged.

As for MISSING, let's take it baby steps. We have enough space in
vm_fault_reason for UFFD_MISSING if we'd want to pull handle_userfault()
from shmem and hugetlb.
 
> If it's me, I'd simply export handle_userfault()..  I confess I still don't
> know why exporting it is a problem, but maybe I missed something.

It's not only about export, it's also about not requiring ->fault()
methods for pte-mapped memory call handle_userfault().

> Only my two cents.  Feel free to go with whatever way you prefer.
> 
> Thanks,
> 
> -- 
> Peter Xu
> 

-- 
Sincerely yours,
Mike.