From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793089917; b=ov/yfq2KFf7CE9oneyt/0H593y+QZDH0Jn9JNke0HNgWfZeTNW1S0pbI8i/17+Ie3tXu BXwbY11LmNfcBT8yQtnjzRI6AeGMqZmctqoTsAlEyL57O//jh4p1urfgcHshIpdys3ksm fB6QQ7O46em6tHmpVIwlUaXYG0VtwJshV8n/ggKewFicmNoz4U3sIxQmHy3Nf6H4c4L/A bb2RhlGt+OYcPyjGQt4vXPcgmniati3tCA2VmDW0MmrfXm1R+9bs2awWEuTaAnARM1vW3 rl7zvWNg/4I3FD89Vh1Q5KRViKBihI7a2eGqn50olQCsLNhEeJw/W+nosuRVaho/2wA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793089917; h=Received:DKIM-Signature:Date:From:To:Cc:Subject:Message-ID: References:MIME-Version:Content-Type:Content-Disposition:In-Reply-To; bh=JYDAxb/IKfi0ONAEetWSuyzn9W1ajE3cX6VVYTxdndk=; b=5jBLRTO4Cd6oDUZj5hEGXow4hGqQVUPTPAR+RwQKQJKUA0wfR4VPNL2UfQ38Bu2BQY7a klQtx3uC10X9V6I1YyIZCyrgY3AQAXRH7FW3Wwm+PErRV8Bb5lG8DknJn184f8bOqaZ6W qJtuMzhl2Grav4PuaCNBzsqORWQFdX57oVzMeJ8H+U0+q71tHhbc5QkQXEjPnV8uzrkha J/LFdnBN6ocYveV3XbHxWZ86w/sP/aQswr85Olry/fENrSVcWUHQqqvJ3fJVCxtWG9MuU dpHgMFDpjEGWDCIgVrebreR7WsjHs6dkQxpnUVxFA3FrFblRHi6xavfomacDfQMVJaw== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=NSyrEmFs; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 63396606A328; Tue, 27 Oct 2026 08:31:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=NSyrEmFs Received: by smtp.kernel.org (Postfix) with ESMTPSA id 30FE51F01825; Tue, 27 Oct 2026 08:31:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793089917; bh=JYDAxb/IKfi0ONAEetWSuyzn9W1ajE3cX6VVYTxdndk=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=NSyrEmFsH2/8hgo8acOSrNLr9Hp/jrECdxCZNkIUtjazyxj1KHY2Z9xTuaK1Lx/+E ZlmTVVItxriaq2TTVszlyqWoslkxBKunusBgrpEBtjqe+GL7/BhgMfvr0j6wiA++no F2KtES+mT9pC9lUi/ceR10V31SP1W8sTkLdb8RXB5buLAYoTKBuslgPDKXcQC7fYaC tmT15Bau5bPVbn3Z1pAmnKqeU5kgzDR3je5XpPQh+1wLiPoL+vicuxqnXgb7RRv82r DFUEPhtPAu/EsK33zZwLXIqaPQ96HHeqGtj52+tHWqHJBcswjRzCsbgunsrigQ97JU n0Iy4vbd890bA== Date: Tue, 27 Oct 2026 08:31:40 +0000 From: "Lorenzo Stoakes (ARM)" To: "David Hildenbrand (Arm)" Cc: Andrew Morton , Johannes Weiner , Michal Hocko , Qi Zheng , Shakeel Butt , Kairui Song , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Baoquan He , Baolin Wang , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Kees Cook , Peter Zijlstra , Jann Horn , Pedro Falcato , Hugh Dickins , Pasha Tatashin , Pratyush Yadav , "Matthew Wilcox (Oracle)" , Jan Kara , Zi Yan , Lance Yang , Rik van Riel , Harry Yoo , Alistair Popple , Nico Pache , Ryan Roberts , Dev Jain , Usama Arif , Kiryl Shutsemau , Matthew Brost , John Hubbard , Peter Xu , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Oscar Salvador , Minchan Kim , Muchun Song , Xu Xin , Chengming Zhou , Chris Li , Kemeng Shi , Nhat Pham , Youngjun Park , Gregory Price , Joshua Hahn , Rakie Kim , Ying Huang , Jason Gunthorpe , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, linux-cxl@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH 25/46] mm/userfaultfd: wake waiters when registration modes are removed Message-ID: References: <20261027-fixes_core-v1-0-6f74bec9a5bb@kernel.org> <20261027-fixes_core-v1-25-6f74bec9a5bb@kernel.org> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261027-fixes_core-v1-25-6f74bec9a5bb@kernel.org> On Tue, Oct 27, 2026 at 12:17:45AM +0100, David Hildenbrand (Arm) wrote: > UFFDIO_REGISTER can replace MISSING or MINOR tracking without waking > faults that are already queued. For example, switching an anonymous > range from MISSING to WP leaves a missing-fault waiter asleep until > userspace explicitly wakes it. I assume this is something people basically never do because you'd think it'd be a pretty obvious bug report right? > > When a registration change removes a UFFD mode, wake waiters in the > affected interval after updating the VMA. The VMA is write-locked at > this point, so no new faults can queue under the old modes and woken > threads cannot retry until mmap_write_unlock(). Waiters for retained > modes in the same interval may also wake and refault. Oh good lord. > > The hang reproduces for MISSING-to-WP and MINOR-to-MISSING transitions, > with both read and unread fault events. > > Fixes: 63b2d4174c4a ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl") > Cc: stable@vger.kernel.org > Signed-off-by: David Hildenbrand (Arm) In the words of Metallica, the more I see, the less, the less I believe... Reviewed-by: Lorenzo Stoakes (ARM) > --- > mm/userfaultfd.c | 20 ++++++++++++++++++++ > 1 file changed, 20 insertions(+) > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c > index 2122676c4220f..710e913c73bfa 100644 > --- a/mm/userfaultfd.c > +++ b/mm/userfaultfd.c > @@ -2270,6 +2270,9 @@ static struct vm_area_struct *userfaultfd_clear_vma(struct vma_iterator *vmi, > return ret; > } > > +static void wake_userfault_range(struct userfaultfd_ctx *ctx, > + unsigned long start, unsigned long end); > + > /* Assumes mmap write lock taken, and mm_struct pinned. */ > static int userfaultfd_register_range(struct userfaultfd_ctx *ctx, > struct vm_area_struct *vma, > @@ -2287,6 +2290,8 @@ static int userfaultfd_register_range(struct userfaultfd_ctx *ctx, > prev = vma; > > for_each_vma_range(vmi, vma, end) { > + const bool any_mode_removed = vma->vm_flags & __VM_UFFD_FLAGS & ~vm_flags; > + > cond_resched(); > > VM_WARN_ON_ONCE(!vma_can_userfault(vma, vm_flags, wp_async)); > @@ -2332,6 +2337,10 @@ static int userfaultfd_register_range(struct userfaultfd_ctx *ctx, > */ > userfaultfd_set_ctx(vma, ctx, vm_flags); > > + /* Retry faults for removed modes while the VMA is write-locked. */ > + if (any_mode_removed) > + wake_userfault_range(ctx, start, vma_end); > + > if (vma_is_hugetlb(vma) && uffd_disable_huge_pmd_share(vma)) > hugetlb_unshare_all_pmds(vma); > > @@ -3648,6 +3657,17 @@ static __always_inline void wake_userfault(struct userfaultfd_ctx *ctx, > __wake_userfault(ctx, range); > } > > +static void wake_userfault_range(struct userfaultfd_ctx *ctx, > + unsigned long start, unsigned long end) > +{ > + struct userfaultfd_wake_range range = { > + .start = start, > + .len = end - start, > + }; > + > + wake_userfault(ctx, &range); > +} > + > static __always_inline int validate_unaligned_range( > struct mm_struct *mm, __u64 start, __u64 len) > { > > -- > 2.43.0 > -- Cheers, Lorenzo