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 D7FDD24C68D for ; Wed, 5 Mar 2025 15:02:55 +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=1741186978; cv=none; b=Zdjq+B1GP6NEVRIbUhq4SCh8CCFywHgFekL1R2r4l5k11zP6qUFZjSE6wIYvww9KdfO5kcWxPckIZ3GO1naSny2aymgdIXKu9Ycsb0U5Qsem3W1QlZUk+1C+1G8K0pvZDKTXVC4oteRQ4AtFODqRTeYhzlBMQSKo1EO70ZgCmPM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1741186978; c=relaxed/simple; bh=rwxs2qHmdeQKe1U8LtwGFdJo7B87r155p+txMieverQ=; h=Message-ID:Date:MIME-Version:Subject:To:References:From: In-Reply-To:Content-Type; b=X/15uvKZ5pgGYU3rQEGSi19ItiXYs7MomHAy5iRjaEVhVkSgvOe+pvw2NVv+RRNXQGIGDHfex0sxblBq1Z0C3+Se0+1i17R8F4AeuDLlJ/GbHAyQOPNZCuDb9Fq634oyvw7s+eQ5ADhjukkxnFxvAzZ4zRmVjTjWDITGI9ezXqQ= 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 9535FFEC; Wed, 5 Mar 2025 07:03:08 -0800 (PST) Received: from [10.174.36.147] (unknown [10.174.36.147]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 065973F673; Wed, 5 Mar 2025 07:02:52 -0800 (PST) Message-ID: <3c3f3cfe-9fa7-41d7-9759-cc67306f13f5@arm.com> Date: Wed, 5 Mar 2025 20:32:49 +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: [QUESTION] Plain dereference and READ_ONCE() in fault handler To: David Hildenbrand , willy@infradead.org, ziy@nvidia.com, hughd@google.com, ryan.roberts@arm.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20250305102159.96420-1-dev.jain@arm.com> <8477d9ec-b9ce-4a3d-b61f-1bd44d3360a5@redhat.com> Content-Language: en-US From: Dev Jain In-Reply-To: <8477d9ec-b9ce-4a3d-b61f-1bd44d3360a5@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 05/03/25 4:16 pm, David Hildenbrand wrote: > On 05.03.25 11:21, Dev Jain wrote: >> In __handle_mm_fault(), >> >> 1. Why is there a barrier() for the PUD logic? > > Good question. It was added in > > commit a00cc7d9dd93d66a3fb83fc52aa57a4bec51c517 > Author: Matthew Wilcox > Date:   Fri Feb 24 14:57:02 2017 -0800 > >     mm, x86: add support for PUD-sized transparent hugepages > > Maybe it was an alternative to performing a READ_ONCE(*vmf.pud). > > Maybe it was done that way, because pudp_get_lockless() does not exist. > And it would likely not be required, because on architectures where > ptep_get_lockless() does some magic (see below, mostly 32bit), PUD THP > are not applicable. Thanks for your reply David. > > >> 2. For the PMD logic, in the if block, we use *vmf.pmd, and in the >> else block >>     we use pmdp_get_lockless(); what if someone changes the pmd just >> when we >>     have begun processing the conditions in the if block, fail in the >> if block >>     and then the else block operates on a different pmd value. >> Shouldn't we cache >>     the value of the pmd and operate on a single consistent value >> until we take the >>     lock and then finally check using pxd_same() and friends? > > The pmd_none(*vmf.pmd) is fine. create_huge_pmd() must be able to deal > with races, because we are not holding any locks. I had a mental hiccup, yes we don't need the cached value even before the if block, as the relevant path will eventually check after taking the lock. I was thinking of all sorts of weird races. > > We only have to use pmdp_get_lockless() when we want to effectively > perform a READ_ONCE(), and make sure that we read something "reasonable" > that we can operate on, even with concurrent changes. (e.g., not read a > garbage PFN just because of some concurrent changes) Oh I just looked at the arm64 definition and assumed ptep_get_lockless() == READ_ONCE() :) Now this makes sense. So I am guessing that we can still get away with a *vmf.pmd on 64-bit arches? A separate question: When taking the create_huge_pmd() path with a read fault and after taking the pmd lock, why shouldn't we check with pmd_none(pmdp_get_lockless(vmf.pmd)) instead of plain *vmf.pmd...surely now we must load the actual correct value from memory since we are committing into mapping the huge zero folio? And after looking somewhat more, why even is a pmd_none(*pmd) there in set_huge_zero_folio()...it should be the responsibility of the caller to verify this? Any caller will just assume that it got the huge zero folio mapped so this check should be redundant. > > We'll store the value in vmf.orig_pmd, on which we'll operate and try to > detect later changes using pmd_same(). So we really want something > consistent in there. > > See the description above ptep_get_lockless(), why we cannot simply do a > READ_ONCE on architectures where a PTE cannot be read atomically (e.g., > 8 byte PTEs on 32bit architecture). >