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 238F74A3408; Tue, 12 May 2026 08:42:20 +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=1778575341; cv=none; b=Q+qJv9VOkKyb+yrwel0GTptbd4b2K4HtiSaO9U5zA+swF2O18C+qlGOKPTmaLLRCD08JTc1JHzpqdD3P1JQ5ek8BA4lCCcfLTXkgJxJpEZEmqpp6ASXIQQwAbysC1cuDMyJqEesbnt87cyWFSpW9neXRXjXOsNvKDHs/FMfNc2Q= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778575341; c=relaxed/simple; bh=ljMVN0mSJd0KTMjrcegOkqI698VOfIwxmfgsHJ7qRXc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=MnrTzu8tNxezML67KSJsFbaqlITBdjHXwO6S6IBK5oUcKtvRlQ2SuQWA2CLU40ssKmMAVxP/W/oAvVUHMFms5ft26Poau8P1mH2DqSvZQDsfxmx72kN2m0QNJ+PNHNJxpii80I0wS73Se3b/6J6bcByUBXnCjxdFeUftROVgsgc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=aXarLmuQ; 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="aXarLmuQ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D0AD5C2BCB0; Tue, 12 May 2026 08:42:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778575340; bh=ljMVN0mSJd0KTMjrcegOkqI698VOfIwxmfgsHJ7qRXc=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=aXarLmuQ5ZFeOPSU6FhyRT4yq+oRCm5H7/kGva7h7c7LxWbIU9kmR6BUDWhapDUzH FDOPx1WtzILPuT68FbOg95T+Le8esLE8B4CuCGLvXh7IUSCWolvsrnNIdEVCNUA3GS HmGFWvPQw4rtOPuWuuvejg41lAKu/JAP65G+JcK9ufm6scgCxP+KCoTAwOIMgtiEx4 E7ijKwL9qJTTVpAnq7cTeyd4VqVa2xoJPEZuclth0mKK/EViKcX7Ek9Ds5LzGKisO7 JqO3VwtI/KRB75BnyN1Lrure/YkgfeeHIeAC75tZP64qgkpEivZLjyA7WNLweI4LmH 9aX3zDFYIbKeQ== Message-ID: <563bb216-c270-4711-adda-b91484af40dc@kernel.org> Date: Tue, 12 May 2026 10:42:14 +0200 Precedence: bulk X-Mailing-List: linux-kselftest@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/3] mm/hmm: Add hmm_range_fault_unlockable() for mmap lock-drop support To: Stanislav Kinsburskii , kys@microsoft.com, Liam.Howlett@oracle.com, akpm@linux-foundation.org, decui@microsoft.com, haiyangz@microsoft.com, jgg@ziepe.ca, corbet@lwn.net, leon@kernel.org, longli@microsoft.com, ljs@kernel.org, mhocko@suse.com, rppt@kernel.org, shuah@kernel.org, skhan@linuxfoundation.org, surenb@google.com, vbabka@kernel.org, wei.liu@kernel.org Cc: linux-doc@vger.kernel.org, linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org References: <177759835313.221039.2807391868456411507.stgit@skinsburskii-cloud-desktop.internal.cloudapp.net> <177759840859.221039.13065406062747296947.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 > + for (; addr < end; addr += PAGE_SIZE) { > + vm_fault_t ret; > + > + ret = handle_mm_fault(vma, addr, fault_flags, NULL); > + > + if (ret & (VM_FAULT_RETRY | VM_FAULT_COMPLETED)) { > + /* > + * The mmap lock has been dropped by the fault handler. > + * Record the failing address and signal lock-drop to > + * the caller. > + */ > + *hmm_vma_walk->locked = 0; > + hmm_vma_walk->last = addr; > + return -EAGAIN; Okay, so we'll return straight from hmm_vma_fault() to hmm_vma_handle_pte()/hmm_vma_walk_pmd() -> walk_page_range() machinery. Hopefully we don't refer to the MM/VMA on any path there? It would be nicer if the hmm_vma_fault() could be called by the caller of walk_page_range(), but that's tricky I guess, as hmm_vma_fault() consumes the walk structure and requires the vma in there. Note: am I wrong, or is hmm_vma_fault() really always called with required_fault=true? > + } > + > + if (ret & VM_FAULT_ERROR) > return -EFAULT; > + } > return -EBUSY; > } > > @@ -566,6 +585,17 @@ static int hmm_vma_walk_hugetlb_entry(pte_t *pte, unsigned long hmask, > if (required_fault) { > int ret; > > + /* > + * Faulting hugetlb pages on the unlockable path is not > + * supported. The walk framework holds hugetlb_vma_lock_read > + * which must be dropped before handle_mm_fault, but if the > + * mmap lock is also dropped (VM_FAULT_RETRY), the vma may > + * be freed and the walk framework's unconditional unlock > + * becomes a use-after-free. > + */ > + if (hmm_vma_walk->locked) > + return -EFAULT; Just because it's unlockable doesn't mean that you must unlock. Can't this be kept working as is, just simulating here as if it would not be unlockable? -- Cheers, David