From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=217.140.110.172 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794493752; b=GeZVIHBO1vi2zIDkOqYaLdcURFhJYyC+4JMUhVsI12E3CyIxcMK/n5aXwtUmKPl1npgz agAdyItCw2rpjJKfCnNB/kVOTovLYcXiboh6/7yYjhYbeWOiJ8MGy933W9UVUhmMtBK/w EoqMKgsXRNkMsC1zCTOnXDvsCoUV3Ae4HxD1E7i+TcoNgHzKHyImcaLZws7PZzjSRwkhr XvsYeKeESkxbST2Dk9asJohMsiyeQwVZD4Rhn9pV8z4nzL+L5pYlGVIeQ7ZvD2aP/21/U c3j2d0GMR6HHpH+AAIAXXdXW6IY5TZv/4OzU3Wt5/j2hpZY67UzzoEVwREYVar93gLA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794493752; h=DMARC-Filter:Received:Received:DKIM-Signature:Date:From:To:Cc: Subject:Message-ID:References:MIME-Version:Content-Type: Content-Disposition:In-Reply-To; bh=sRazLDIEc7CMUp/9vgq2YR+tcffN4fGN/wRdKhy4qfE=; b=dCAbOXLlPeS8dtKFGuweVkTxCpWU/ERxP1fVp0abB59N7ZVvsZqxYZXZaIhAichGQDFr 8Mm44LW3LYsuC4dk7uX4ybpriGXS8Akh+W1UHJ/Wvdkvi22xF2eUg493c0ebQqqPPCB+k cPm2/2u4AoqnRR6SeTIa+zGkGRVZM+Uqte3ia0pSWcGzA6xWbjeT6oh5oaEWVLjdCsE4o YEy8pj4c1yvIk508aX1qFTtQsjtKacoV+EjBbjKzWmOPynODvXH/Iz4nzi5UEBp9Dcxy8 PZj2jPjUoWTUd6ifZUsHpULqdUAMQFCs9v9vgKLPUJqJ1lLGsmbpB3kuqQRKti4j22Q== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=arm.com; spf=pass smtp.mailfrom=arm.com; dkim=pass header.d=arm.com header.i=@arm.com header.a=rsa-sha256 header.s=foss header.b=fby67aXm; arc=none smtp.remote-ip=217.140.110.172 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id E084E606A33D for ; Thu, 12 Nov 2026 14:29:11 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org E084E606A33D Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.a=rsa-sha256 header.s=foss header.b=fby67aXm Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id E07A61477; Thu, 12 Nov 2026 06:29:07 -0800 (PST) Received: from arm.com (usa-sjc-mx-foss1.foss.arm.com [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id D93B83F673; Thu, 12 Nov 2026 06:29:08 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1794493750; bh=XiaNpbbvxm6cKE/zgmNwmyN9+sHlHIuKTGnW432EXr4=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=fby67aXmxYqsYQ+cJC3GcBthEnnJJfdGSIYm40Lcu20zGsDk9qauSSN3TVDH36s+A aizxncjEclC0Gap/jNtf43T4IKOjBghSsjwIFWshSDsu1J7xkfVFbrN8L9ZwyHuEn+ GCODgR3flC+q2PeYAw8JU5AHNQWkEmCxf053CvKo= Date: Thu, 12 Nov 2026 14:29:06 +0000 From: Catalin Marinas To: "Oscar Salvador (SUSE)" Cc: Chris Down , Will Deacon , linux-kernel@vger.kernel.org, kernel-team@meta.com, Mark Rutland , Huang Ying , Andrew Morton , Muchun Song , Oscar Salvador , David Hildenbrand , Mike Kravetz , linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org Subject: Re: [PATCH] arm64: hugetlb: Prevent write fault livelocks on stale TLB entries Message-ID: References: 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: On Wed, Nov 11, 2026 at 10:04:19AM +0100, Oscar Salvador (SUSE) wrote: > On Tue, Nov 10, 2026 at 08:39:02PM -0800, Chris Down wrote: > > In Meta production, we have had tasks get stuck in a fault loop while > > using hugetlb on arm64. During my investigations I found that affected > > tasks share a private hugetlb page. > > > > Looking at the stuck task states, I found the following ordering: > > > > CPU0 CPU1 > > ---- ---- > > read page > > [read only entry cached in TLB] > > write page > > hugetlb_fault() > > hugetlb_wp() > > [reuse folio] > > set_huge_ptep_maybe_writable() > > set_huge_ptep_writable() > > [flush local TLB only] > > > > write page using stale entry > > hugetlb_fault() > > huge_ptep_set_access_flags() > > [entry already writable, no flush] > > retry write using stale entry > > [fault indefinitely. well, that's not ideal!] > > > > The reason is that when a write fault makes a non-contiguous hugetlb > > entry writable, it only invalidates the local TLB. However, another CPU > > can keep the old read-only entry, so its write will fault even though > > the page table already permits it. hugetlb_fault() then finds nothing to > > change and leaves the stale entry in place, so the same write keeps > > going round in circles. > > > > This is particularly bad because, if the kernel is copying to user > > memory, the retry stays inside the kernel and the task can't act on the > > pending signal. > > > > It can even remain unkillable while holding locks across the copy, > > resulting in further failures. For example, a read into a hugetlb > > buffer can hold the pipe mutex and block other users of the pipe until > > the loop ends, causing cascading lockups across the machine. > > > > To fix this, when a write fault finds nothing to change in a > > non-contiguous hugetlb entry, let's invalidate the local entry. > > > > Fixes: cb1fa2e99955 ("arm64, tlbflush: don't TLBI broadcast if page reused in write fault") > > Cc: > > Signed-off-by: Chris Down > > This makes sense to me, and I see that commit cb1fa2e99955 ("arm64, tlbflush: > don't TLBI broadcast if page reused in write fault") only invalidated local tlb > for cont-hugetlb. > Reading the changelog, I guess that was an oversight because I could not find > the reason behind. I think you mean non-contiguous hugetlb here? cb1fa2e99955 switched __ptep_set_access_flags() to local invalidation, used on the non-contiguous hugetlb path. Contiguous hugetlb updates still broadcast through get_clear_contig_flush(). Ordinary contpte mappings also switched to local invalidation but those go through the generic fault handler and its fix_spurious_fault() fallback. That fallback is what the hugetlb path was missing. -- Catalin