From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 689B728CF61; Thu, 15 May 2025 08:47:34 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1747298858; cv=none; b=Sih5NXnnfb6l6iNz8bssMBDeYsv4yp4RfKIJsIF/MKl2WUaoHU/Xm59aY6oOqjIuT68E17c+8QVv4bDaDl375eKSKU+x19CBE+Dsj8puDvA6ff4/q6/hJ+l+xBJ53hRKT/HDonBoWVFPTvCod923BI7rRwfIHwlmoiMOSGV+UXU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1747298858; c=relaxed/simple; bh=Bw4wosFmdzVjrqAaQ4HVzmHqwpE1UTT+LbWM5M1KXAM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=paBpqdFNdANzETSXN7w11TplIN3tiW5M6+4XTmSGb4MB3EXfGQYR+t+GRYs3eBZ//umwxMQDBhoQZ3PBHUtpEPiW8JAmKk5Kn71Fzd0mkK4dxeOkZ7a42XIBbw6ofALrTW12RcABeNvFuWDsO//AY+gIgkrBHQb1s3+58gH8VBY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 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 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 CC1AC14BF; Thu, 15 May 2025 01:47:21 -0700 (PDT) Received: from [10.162.40.26] (K4MQJ0H1H2.blr.arm.com [10.162.40.26]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 45DCD3F673; Thu, 15 May 2025 01:47:30 -0700 (PDT) Message-ID: <35ef7691-7eac-4efa-838d-c504c88c042b@arm.com> Date: Thu, 15 May 2025 14:17:27 +0530 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: [PATCH] arm64: Check pxd_leaf() instead of !pxd_table() while tearing down page tables To: David Hildenbrand , catalin.marinas@arm.com, will@kernel.org Cc: ryan.roberts@arm.com, anshuman.khandual@arm.com, mark.rutland@arm.com, yang@os.amperecomputing.com, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, stable@vger.kernel.org References: <20250515063450.86629-1-dev.jain@arm.com> <332ecda7-14c4-4dc3-aeff-26801b74ca04@redhat.com> <4904d02f-6595-4230-a321-23327596e085@arm.com> <6fe7848c-485e-4639-b65c-200ed6abe119@redhat.com> Content-Language: en-US From: Dev Jain In-Reply-To: <6fe7848c-485e-4639-b65c-200ed6abe119@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 15/05/25 2:06 pm, David Hildenbrand wrote: > On 15.05.25 10:22, Dev Jain wrote: >> >> >> On 15/05/25 1:43 pm, David Hildenbrand wrote: >>> On 15.05.25 08:34, Dev Jain wrote: >>>> Commit 9c006972c3fe removes the pxd_present() checks because the caller >>>> checks pxd_present(). But, in case of vmap_try_huge_pud(), the caller >>>> only >>>> checks pud_present(); pud_free_pmd_page() recurses on each pmd through >>>> pmd_free_pte_page(), wherein the pmd may be none. >>> The commit states: "The core code already has a check for pXd_none()", >>> so I assume that assumption was not true in all cases? >>> >>> Should that one problematic caller then check for pmd_none() instead? >> >>   From what I could gather of Will's commit message, my interpretation is >> that the concerned callers are vmap_try_huge_pud and vmap_try_huge_pmd. >> These individually check for pxd_present(): >> >> if (pmd_present(*pmd) && !pmd_free_pte_page(pmd, addr)) >>     return 0; >> >> The problem is that vmap_try_huge_pud will also iterate on pte entries. >> So if the pud is present, then pud_free_pmd_page -> pmd_free_pte_page >> may encounter a none pmd and trigger a WARN. > > Yeah, pud_free_pmd_page()->pmd_free_pte_page() looks shaky. > > I assume we should either have an explicit pmd_none() check in > pud_free_pmd_page() before calling pmd_free_pte_page(), or one in > pmd_free_pte_page(). > > With your patch, we'd be calling pte_free_kernel() on a NULL pointer, > which sounds wrong -- unless I am missing something important. Ah thanks, you seem to be right. We will be extracting table from a none pmd. Perhaps we should still bail out for !pxd_present() but without the warning, which the fix commit used to do. > >> >>> >>> If you were able to trigger this WARN, it's always a good idea to >>> include the splat in the commit. >> >> I wasn't able to, it is just an observation from code inspection. > > That better be included in the patch description :) >