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 B53D536B06A for ; Wed, 21 Jan 2026 12:28:52 +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=1768998532; cv=none; b=k5522e6Fii2NuwdwMuuGDV5gfbE80spZhA+mpq8KzLpTuoESHJfTXUEMZWRSldiLX0Ra2Yunr7grqnS5yeyCG9fxsm9+xqRNB2lMUHbBH+L2/eElOsk32FYT/EPIhoTMSTXOxCmXoCvZY9Yc0umLbQcltxPVNLuSZxXt5nAxAXw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768998532; c=relaxed/simple; bh=rD/oyKTE7w7JyTws5uUSUhNwYTZKRqc+wudLFAsBZm0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=fos8yGBvh7AEoFy6oeg+iOS+9YW/5hixp/flF0O+o6t2vxV2HaKYjxv+XTQCk/74mY6UAIODq7b9rPtEkwQNKC29/Y4G+LA8aBTZiqPa2biGo87qkP+DoqM7dFLQkh5q6aWNd+OcEXNaVrsMZgTvkmkPgCXiQfVWt1bwOAeRyHE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Van3gsou; 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="Van3gsou" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 710C7C19425; Wed, 21 Jan 2026 12:28:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1768998532; bh=rD/oyKTE7w7JyTws5uUSUhNwYTZKRqc+wudLFAsBZm0=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Van3gsou9ts4cyfsepozn5LKKuQo2aUwO3UBchz0wUFhx/dkpRKOy+573A3NvmjZh jGezuSTzzTppkqwP0rE4dfyxIgbuAKwyYU1qtUzhXduKYP1SsozyvQ5eLK3M7wW5m9 qLTzpFTuW8sg/028g70fznhb8P5OIF+1scPNLLyJWIDS6wHW5Y/Js2/qQ8SCQ6oHp0 aLXUQgcMUuHKFmiaVgNPJnqApm1XY8plrpbYH61emhS3w9du8UpV4iMMqtMdfrXyXH 4KuBepeH/tHjeMC4+fBWSh8GNJhjEz/44W4kSTIU6tOgfhxIAMz989uYoR9TF5lKHH Ge3oqL3EOMQSw== Message-ID: Date: Wed, 21 Jan 2026 13:28:46 +0100 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] mm/huge_memory: Fix iterator variable usage after swap() To: Lance Yang , zenghongling , Qi Zheng , Muchun Song Cc: linux-mm@kvack.org, dev.jain@arm.com, akpm@linux-foundation.org, ryan.roberts@arm.com, baolin.wang@linux.alibaba.com, npache@redhat.com, linux-kernel@vger.kernel.org, baohua@kernel.org, Liam.Howlett@oracle.com, zhongling0719@126.com, ziy@nvidia.com, lorenzo.stoakes@oracle.com References: <20260121081343.713715-1-zenghongling@kylinos.cn> From: "David Hildenbrand (Red Hat)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 1/21/26 10:25, Lance Yang wrote: > > > On 2026/1/21 16:13, zenghongling wrote: >> The iterator variable 'folio' is swapped with 'prev' in the else >> branch. Using 'folio' after swap() checks the potentially NULL >> 'prev' value, not the original iterator value. >> >> Fix by moving folio_put() call before the swap operation in the >> path where swap() occurs. >> >> Found by: >> ./huge_memory.c:4225:6-11: ERROR: iterator variable bound on line 4178 cannot be NULL > Which tool did find that? A compiler? > Good catch! > > But which tree is your patch based on? > > Seems like that was already fixed in commit 776bde7caf80[1]. The > whole thing deferred_split_scan() was refactored using folio_batch, > so the buggy code with swap(folio, prev) is gone ... > > Ccing Muchun and Qi who fixed that. > > [1] > https://lore.kernel.org/all/59cb6b6fb5ffcff9d23b81890b252960139ad8e7.1762762324.git.zhengqi.arch@bytedance.com/ Right, in commit 776bde7caf80f6af72b087cafe7d9f607b14716d Author: Muchun Song Date: Mon Nov 10 16:17:57 2025 +0800 mm: thp: use folio_batch to handle THP splitting in deferred_split_scan() Which raises the question whether we would want to backport that patch to stable kernels if there was indeed a problem? But: I don't immediately see the problem. If pref is NULL (and folio obviously !+NULL), we'll end up with * pref != NULL * folio == NULL The "if (folio)" check will do nothing, because we defer the freeing to the if (prev) folio_put(prev); later If pref is != NULL (and folio obviously !+NULL), we'll end up with * pref = NULL * folio = NULL The if (folio) and if (prev) handling will care of it all. So ... this pretty much looks like working as expected? -- Cheers David