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] [day] [month] [year] [list]

Message-ID: <aSYBrH_xfMfs6yDW@x1.local>
Date: Tue, 25 Nov 2025 14:21:16 -0500
From: Peter Xu <peterx@...hat.com>
To: Mike Rapoport <rppt@...nel.org>
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

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/

> 
> Instead of calling handle_userfault() directly from a specific ->fault()
> implementation introduce new fault reason VM_FAULT_UFFD_MINOR that will
> notify the core page fault handler that it should call
> handle_userfaultfd(VM_UFFD_MISSING) to complete a page fault.

Same.

> 
> Replace a call to handle_userfault(VM_UFFD_MISSING) in shmem and use the

Same.

> 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.

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.

Only my two cents.  Feel free to go with whatever way you prefer.

Thanks,

-- 
Peter Xu