From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 B6DB81A6815 for ; Wed, 22 Apr 2026 07:27:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776842869; cv=none; b=XzHhgXfugj4fClRE7qweAOstX5vuORb5DNFZ4VmTI7jYNvl2ZF4qCtRgywlk71M7e1pnENjbphRWVrDjM2YQ8VjAwW4kP0WKur8przSTwpK5i3rP7Rokad9Zbnjy6c3FERriW5UN34HaDJVg7xGPc/UQAoP5lFov78LRuK9fuZc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776842869; c=relaxed/simple; bh=kyW/H7rmWbDFYKIRV2cP1lpKsHg1XGw394/LBXDvvCE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=gGoaIc/UqjKvtRb1XfBf5QRZDeuSGZlbQr2zVBRmRzgHQsAi17INlX5CBiWPJPrFmy8ICfzvVhFdyXCTi9LdoFBB4YSShmdzGUyLSJQa5pwHZLjQEDs16F+jAQpZHnbaG+8OoVzzv6n5kN8z+vm9lU06CGxicMMZ8OiFy3VwPKg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=LSO7R61w; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="LSO7R61w" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8E814C19425; Wed, 22 Apr 2026 07:27:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1776842869; bh=kyW/H7rmWbDFYKIRV2cP1lpKsHg1XGw394/LBXDvvCE=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=LSO7R61wL+aLO6KbAwe0jWoNWM1FAzOzCMNhg8aS2Yo8Z6Sh2e6ZhiEgV6gGZp+7g TKvjymHtBxm13ODqKbgk/8bN+82kmzGGzou1qqNE8IFrtUtjc1Ov1XSqjTClGDsYki XidPsUY42Fs64GVSKn80bxAWxBhpjxni5k4O/Ta211VYccQpQpyNpfCvZLZL0f/or1 Sukjag9jvAqv3c941+wWOrAp1H6wCPU01831ivgBotybb7Qoz+CPijLK4zEcLx45KZ IQldhlbIaJrrhBfppaWGoVfa/I35WQo0MMLtRI5zQtXTHHogdNsefN0FtZkdXhvhV7 Ek8K4hCZDCNpQ== Message-ID: <286425bc-e2a2-4e29-abdf-666e798e6cf4@kernel.org> Date: Wed, 22 Apr 2026 09:27:44 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] mm/hmm: Add userfaultfd support to fault handling To: Stanislav Kinsburskii , jgg@ziepe.ca, leon@kernel.org, akpm@linux-foundation.org Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <177499585916.169073.8313365481186071940.stgit@skinsburskii-cloud-desktop.internal.cloudapp.net> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/1/26 00:24, Stanislav Kinsburskii wrote: > Add support for userfaultfd-enabled VMAs to the HMM framework. > > Extract fault handling logic into hmm_handle_mm_fault() to handle both > regular and userfaultfd-backed mappings. The implementation follows > fixup_user_fault() for handling VM_FAULT_RETRY and VM_FAULT_COMPLETED, with > a key difference: instead of retrying or moving forward respectively, > return -EBUSY after reacquiring mmap_read_lock. Since the lock was > released, the VMA could have changed, so defer retry logic to the caller. > > This approach is inefficient for userfaultfd-backed VMAs, as HMM can only > populate one page at a time, but keeps the framework simple by avoiding > complex retry logic within HMM itself. > > Signed-off-by: Stanislav Kinsburskii > --- > mm/hmm.c | 40 ++++++++++++++++++++++++++++++++++++---- > 1 file changed, 36 insertions(+), 4 deletions(-) > > diff --git a/mm/hmm.c b/mm/hmm.c > index f6c4ddff4bd6..d04d68e21473 100644 > --- a/mm/hmm.c > +++ b/mm/hmm.c > @@ -59,6 +59,35 @@ static int hmm_pfns_fill(unsigned long addr, unsigned long end, > return 0; > } > > +static int hmm_handle_mm_fault(struct vm_area_struct *vma, > + unsigned long addr, > + unsigned int fault_flags) > +{ > + int ret; > + > + if (userfaultfd_missing(vma)) { > + struct mm_struct *mm = vma->vm_mm; > + > + fault_flags |= FAULT_FLAG_ALLOW_RETRY | > + FAULT_FLAG_USER; > + > + ret = handle_mm_fault(vma, addr, fault_flags, NULL); > + > + if (ret & (VM_FAULT_COMPLETED | VM_FAULT_RETRY)) { > + mmap_read_lock(mm); > + return -EBUSY; > + } > + > + if (ret & VM_FAULT_ERROR) > + return vm_fault_to_errno(ret, 0); > + } else { > + ret = handle_mm_fault(vma, addr, fault_flags, NULL); > + if (ret & VM_FAULT_ERROR) > + return vm_fault_to_errno(ret, 0); > + } I'm surprised that there is userfaultfd_missing() logic required here at all. What prevents us from always calling handle_mm_fault() in a way + handling return values, such that we will just do the right thing independent of userfaultfd_missing()? -- Cheers, David