From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B174D3932C8; Tue, 25 Aug 2026 10:37:44 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787654266; cv=none; b=pmIj5nMwqz7n5XcpJIPwwzeDlP+z3+ALUtH2JKtxyA3CLgF8mEBDEVn+tXOvi2p67VDdm4JwV85wYZCahyyls++oAMU+ma0c7sX/Okkcv863GePvSb6fMokl0dUBY3A/Z+/vkp+QifO2y1OaJuKP3T8nLye7sAkVKj2mv7CVJLU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787654266; c=relaxed/simple; bh=6wM71OktoHpoNMwnvS4y5Ps47GC3beCdecG1N+mXMK8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=pbdnpBfK44XznCC1NJMsO1ruPg94OihR9ZN3qk6dVsI3V4x5Zuvw/BUXCllE0YZsq1EPpXooIVRXIxFb2DoXF61XW18/V5PGoV3GxlH5/+Xh9pSrhS5KX9kXqmP30ZLqeFFm2yjslYthatIe8Rgpel6TWtHHcgGd7KAXugHionM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=BBdrmQL9; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="BBdrmQL9" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D788D1F000E9; Tue, 25 Aug 2026 10:37:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1787654264; bh=VlrD/QAeSxGpKzZmYIBSyb0ejvsuQwHLEbsOamW6WOQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=BBdrmQL99+j2LXsw9s0xPLciJ2E/I2z+Gr4SojHcMBhz1QUD9dzjGPthOkVvY6Oxc 177CwdrzU/aU+5tVOPw/SVnbst1ZAAyvPMvY+JkAL0Bkbinj46KhuzpTP43sGrQ9nW /Hb4CgTn/sT1Rlo8sGRLQW0/cnGFbXzoX1UknZY2DoAmTfC2sgb64vcrVWGNq/spuv MWAsgIxfIZC/kzdgk5PUmfTy1XJHDn6F/CN5ZyLEDWAU8KAVfaGguZW7yY/ztfHmmQ 7PAQFcZVZqXy5+R+WXQyFRo47k6MCK8CMonD1iX33XXR2k7GpExAUQgVddyB9IyYu3 rMCaMoa2soSkg== Date: Tue, 25 Aug 2026 13:37:30 +0300 From: Mike Rapoport To: "Lorenzo Stoakes (ARM)" Cc: Andrew Morton , David Hildenbrand , Baolin Wang , Barry Song , Dev Jain , Hugh Dickins , Jann Horn , Jason Gunthorpe , John Hubbard , Jonathan Corbet , Lance Yang , "Liam R. Howlett" , Masami Hiramatsu , Mathieu Desnoyers , Michal Hocko , Muchun Song , Nico Pache , Oscar Salvador , Pedro Falcato , Peter Xu , Ryan Roberts , Shakeel Butt , Shuah Khan , Steven Rostedt , Suren Baghdasaryan , Usama Arif , Vlastimil Babka , Zi Yan , linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org Subject: Re: [PATCH 5/6] userfaultfd: decouple fault reason from VMA flags Message-ID: References: <20260823-uffd-vm-flags-v1-v1-0-3086981b33cf@kernel.org> <20260823-uffd-vm-flags-v1-v1-5-3086981b33cf@kernel.org> Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Mon, Aug 24, 2026 at 05:28:29PM +0100, Lorenzo Stoakes (ARM) wrote: > On Sun, Aug 23, 2026 at 03:17:42PM +0300, Mike Rapoport (Microsoft) wrote: > > Introduce enum uffd_reason to define reasons for user faults rather than > > overload VM_UFFD_* VMA flags for that. > > > > Using a dedicated enum makes the code clearer and decoupling the fault > > reason from VMA flags clears the way for moving the uffd mode bits out > > of VMA namespace. > > > > No functional change. > > > > Assisted-by: copilot:claude-opus-4.6 > > Signed-off-by: Mike Rapoport (Microsoft) > > --- > > include/linux/userfaultfd_k.h | 16 ++++++++++++++-- > > include/uapi/linux/userfaultfd.h | 6 +++--- > > mm/huge_memory.c | 6 +++--- > > mm/hugetlb.c | 10 +++++----- > > mm/memory.c | 10 +++++----- > > mm/shmem.c | 4 ++-- > > mm/userfaultfd.c | 30 +++++++++++++++--------------- > > 7 files changed, 47 insertions(+), 35 deletions(-) > > > > diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h > > index 45355bdb4ec7..f401623f315d 100644 > > --- a/include/linux/userfaultfd_k.h > > +++ b/include/linux/userfaultfd_k.h > > @@ -9,6 +9,18 @@ > > #ifndef _LINUX_USERFAULTFD_K_H > > #define _LINUX_USERFAULTFD_K_H > > > > +#include > > + > > +/* Fault reason #PF handler passes to handle_userfault() */ > > +enum uf_reason { > > + USERFAULT_MISSING = BIT(0), > > + USERFAULT_MINOR = BIT(1), > > + USERFAULT_RWP = BIT(2), > > + USERFAULT_WP = BIT(3), > > +}; > > Hmm your commit message says uffd_reason, uf_reason makes me think of the > character Ulf from House of the Dragon. But not uffd. So as per David let's > rename it :) > > I'm also not sure if an enum is the right thing for flag values? I'll ask LLM why it chose it :) > Anything that is parameterised by enum uffd_reason that combines flags will > break any switch statement in there and yada yada. > > I wonder if better just as #define's + unsigned long or something? > > Or you could do (and this leads to nicer stuff later): > > enum uffd_reason { > USERFAULT_MISSING_BIT = 0, > USERFAULT_MINOR_BIT = 1, > USERFAULT_RWP_BIT = 2, > USERFAULT_WP_BIT = 3, > }; > > #define USERFAULT_MISSING BIT(USERFAULT_MISSING_BIT) > etc. Looks over-engineered to me tbh, if we drop an enum, I'd just #define FLAG (1 << SHIFT) and call it a day. Also see below about aligning with uABI flags. > > @@ -168,9 +168,9 @@ struct uffd_msg { > > > > /* flags for UFFD_EVENT_PAGEFAULT */ > > #define UFFD_PAGEFAULT_FLAG_WRITE (1<<0) /* If this was a write fault */ > > -#define UFFD_PAGEFAULT_FLAG_WP (1<<1) /* If reason is VM_UFFD_WP */ > > -#define UFFD_PAGEFAULT_FLAG_MINOR (1<<2) /* If reason is VM_UFFD_MINOR */ > > -#define UFFD_PAGEFAULT_FLAG_RWP (1<<3) /* If reason is VM_UFFD_RWP */ > > +#define UFFD_PAGEFAULT_FLAG_WP (1<<1) /* If reason is uffd-wp */ > > +#define UFFD_PAGEFAULT_FLAG_MINOR (1<<2) /* If reason is uffd-minor */ > > +#define UFFD_PAGEFAULT_FLAG_RWP (1<<3) /* If reason is uffd-rwp */ > > Is it worth retaining the same bit indexes as the reasons? > > Reasons: > > Bit number > MINOR 0 > RWP 1 > WP 2 > > Page fault flags: > > Bit number > MINOR 2 > RWP 3 > WP 1 If we go this way, than it must be #define USERFAULT_MINOR UFFD_PAGEFAULT_FLAG_MINOR so we won't need to keep them in sync explicitly. With a caveat of USERFAULT_MISSING that is expressed as "no flags in uffd_msg" :) > > @@ -2607,7 +2607,7 @@ static inline void msg_init(struct uffd_msg *msg) > > static inline struct uffd_msg userfault_msg(unsigned long address, > > unsigned long real_address, > > unsigned int flags, > > - unsigned long reason, > > + enum uf_reason reason, > > unsigned int features) > > { > > struct uffd_msg msg; > > @@ -2629,11 +2629,11 @@ static inline struct uffd_msg userfault_msg(unsigned long address, > > */ > > if (flags & FAULT_FLAG_WRITE) > > msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WRITE; > > - if (reason & VM_UFFD_WP) > > + if (reason & USERFAULT_WP) > > msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WP; > > - if (reason & VM_UFFD_RWP) > > + if (reason & USERFAULT_RWP) > > msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_RWP; > > - if (reason & VM_UFFD_MINOR) > > + if (reason & USERFAULT_MINOR) > > msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_MINOR; > > With matching flags and unsigned long you could do > > msg.arg.pagefault.flags |= reason; > > I think? Almost: msg.arg.pagefault.flags |= (reason & ~USERFAULT_MISSING); And define USERFAULT_MISSING as (1 << 0) with a comment why it's fine. I don't feel strongly about it, but my preference is to define reason flags independently of UFFD_PAGEFAULT_FLAGs and keep the ifs here. > > @@ -2793,14 +2793,14 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx, > > * If VMA has UFFD WP faults enabled and WP fault, wait for userspace to > > * resolve the fault. > > */ > > - if (!pte_write(ptent) && (reason & VM_UFFD_WP)) > > + if (!pte_write(ptent) && (reason & USERFAULT_WP)) > > I wonder if you could actually > > You do this quite a lot and they read a bit horribly with the && and & on the > same sight-line. With the changes to the enum proposed above you could do: > > if (!pte_write(ptent) && test_bit(reason, USERFAULT_WP_BIT)) I find && and & perfectly readable and adding _BIT defines looks really excessive to me. > > @@ -2835,7 +2835,7 @@ static inline unsigned int userfaultfd_get_blocking_state(unsigned int flags) > > * fatal_signal_pending()s, and the mmap_lock must be released before > > * returning it. > > */ > > -vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason) > > +vm_fault_t handle_userfault(struct vm_fault *vmf, enum uf_reason reason) > > Hmm what was the 'reason' here before? The flags? Maybe more reason (no pun > intended) to keep the values the same? The 'reason' before was a VM_UFFD_SOMETHING, we really can't keep the values the same, but we surely can keep it unsigned long. > -- > Cheers, Lorenzo -- Sincerely yours, Mike.