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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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: EFBC716000D X-Stat-Signature: 56zk9hpnfdcrrt6eae85oz8n3tktfgn4 X-Rspam-User: X-HE-Tag: 1787816551-763277 X-HE-Meta: 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 +z9brNHX 7EyqvCdqm21kYdxlX489seK/7r7NJV459bpQu/iQbar/9RmQtJGw0U+21ioJYCmcuQW3DGGDE4X6d6H8pxDdh6/cHUkHLpZashzm54XK7dOybhD/+b03gmfTCif9jSTwR4FF6y5QcR6g7GtgdpUBMpkPgu33Gx81w/e+XgeL/4viUarXB4QlBDLI6R0+s4kfPipW+HsxTF6sRnTJeH8G49p9i3tkid+sTICSkIUeJqQr9huZT+Qg7N6KPK+P+dbCR5RsGO4wVk2MqfrassyZqICJ8Pg== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Aug 25, 2026 at 02:00:10PM +0100, Lorenzo Stoakes (ARM) wrote: > On Tue, Aug 25, 2026 at 01:37:30PM +0300, Mike Rapoport wrote: > > 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 :) > > I mean you then introduce the same flags again seemingly with different > names as #define's in the next patch... having several sets of flags with > subtly different names seems unwise. The names are important, the values are not. There are two cases that currently use the same VMA_UFFD_* flags: * the way VMA is registered with uffd, i.e. the 'mode' part * the type of the user fault that the generic #PF handler passes to handle_userfault() They are related, a fault in a VMA that was registered as MISSING will never pass MINOR to handle_userfault(), but I think it'll be actually clearer to separate them semantically, so that when you read a call site of handle_userfault() it is clear what type of the fault it is and when you parse userfaultfd code you see what modes user wanted for a VMA. > > > 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. > > See review on 6/6, I'm confused actually why we have several sets of these > flags... I can see that ;-) > But in general it seems like these flags (in one form or another) are being > repeatedly referenced, so it's not really over-engineering I don't think to > abstract some of that. Again, the bit numbers do not matter, they are the same because it's easy to count from 0. I can make one of those count backwards if it helps :) > Maybe can be in wrappers that make it nicer. But really the issue is the > duplication in modes/reasons/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" :) > > Ugh. Yeah, and the PAGEFAULT_FLAG numbers are set in stone because it's uABI. > > > > > > 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. > > And also modes... Again I think fixing that mess somehow is the better way forward. Can you elaborate? > > > > @@ -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. > > Discussed in sub-thread. We'll agree to disagree I suppose. Yes, we will :) > > > > @@ -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. > > I notice the 'mode' which is not the same as the 'reason' is an unsigned > int in 6/6... Didn't you suggest to make 'reason' an unsigned int as well? 'mode' in 6/6 is an unsigned int because if it were an enum it'd require #include in mm_types.h, see the commit message there. > -- > Cheers, Lorenzo > -- Sincerely yours, Mike.