From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 92497EF5865 for ; Mon, 23 Feb 2026 04:40:52 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 9B3BA6B0088; Sun, 22 Feb 2026 23:40:51 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 961EC6B0089; Sun, 22 Feb 2026 23:40:51 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 86DF66B008A; Sun, 22 Feb 2026 23:40:51 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0013.hostedemail.com [216.40.44.13]) by kanga.kvack.org (Postfix) with ESMTP id 72F406B0088 for ; Sun, 22 Feb 2026 23:40:51 -0500 (EST) Received: from smtpin14.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay05.hostedemail.com (Postfix) with ESMTP id 9F1D45B282 for ; 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imf04.hostedemail.com; dkim=none; dmarc=pass (policy=none) header.from=arm.com; spf=pass (imf04.hostedemail.com: domain of dev.jain@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=dev.jain@arm.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1771821649; a=rsa-sha256; cv=none; b=7k7snpkiuoLvHedASaLJljEfPHMFe+2g4tefUSC8N/nYK1PooKhpkcKYA4902PUVG4y264 rgdZFqYrJcjhEpWkS/ijlmdfX97cuzzvpaRPcPXptJYNNT4YSrIcmHIJBeglCAObQQ+zhz f3KW4Hbn3/nDuaLaKB/N9NeZHsCg4ws= 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 2A03D339; Sun, 22 Feb 2026 20:40:41 -0800 (PST) Received: from [10.164.19.28] (unknown [10.164.19.28]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 118753F59E; Sun, 22 Feb 2026 20:40:43 -0800 (PST) Message-ID: Date: Mon, 23 Feb 2026 10:10:41 +0530 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] khugepaged: convert redundant check to WARN_ON To: Andrew Morton Cc: david@kernel.org, lorenzo.stoakes@oracle.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, baohua@kernel.org, lance.yang@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260219054827.4111334-1-dev.jain@arm.com> <20260219143327.7693e4b1474a337821e4c637@linux-foundation.org> Content-Language: en-US From: Dev Jain In-Reply-To: <20260219143327.7693e4b1474a337821e4c637@linux-foundation.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 8BC6E40003 X-Stat-Signature: ykh9dsn4cizwr4b3bp7a1f1asjwhxqza X-Rspam-User: X-HE-Tag: 1771821648-656226 X-HE-Meta: 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 QYo3SOiZ 5KJudjkDsnxYq48nF9Lj10JJmFK9Ux970gJmefXBePMmDJ2puMS6hNS377dC5vSUNyYqlARkykgPd4iRAcZjpgV4Twl4MrR3NamTExwX6ikTHEkoyOSgwmse3uVfFsBpES5ppw3YdckH0DDt/TwX6rGG+wiuGhpUwT6fO7k+AquYhQpBhO0yiAehDiiATYxInCz0zzI0DkM2dMQz+7USdzvK75WptRV5nW3g/tCfxdGozIivgedgw+Is+Tw== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 20/02/26 4:03 am, Andrew Morton wrote: > On Thu, 19 Feb 2026 11:18:27 +0530 Dev Jain wrote: > >> Claim: folio_order(folio) == HPAGE_PMD_ORDER => folio->index == start. >> >> Proof: Both loops in hpage_collapse_scan_file and collapse_file, which >> iterate on the xarray, have the invariant that >> start <= folio->index < start + HPAGE_PMD_NR ... (i) >> A folio is always naturally aligned in the pagecache, therefore >> folio_order == HPAGE_PMD_ORDER => IS_ALIGNED(folio->index, HPAGE_PMD_NR) == true ... (ii) >> thp_vma_allowable_order -> thp_vma_suitable_order requires that the virtual >> offsets in the VMA are aligned to the order, >> => IS_ALIGNED(start, HPAGE_PMD_NR) == true ... (iii) >> >> Combining (i), (ii) and (iii), the claim is proven. >> >> Therefore, convert this to a VM_WARN_ON. >> >> ... >> >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -2000,8 +2000,9 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr, >> * we locked the first folio, then a THP might be there already. >> * This will be discovered on the first iteration. >> */ >> - if (folio_order(folio) == HPAGE_PMD_ORDER && >> - folio->index == start) { >> + if (folio_order(folio) == HPAGE_PMD_ORDER) { >> + VM_WARN_ON(folio->index != start); > It's a bad sad to remove unneeded code by retaining that code and > adding even more code. > > Perhaps add a comment reminding us to remove this altogether at a later > date? But then shouldn't I remove it just now :) try_collapse_pte_mapped_thp is anways going to bail out if such a bug occurs, so nothing goes wrong. In short this check is actually useless, apart from the fact that this lets us spot a bug (which I doubt won't be figured out by other code paths already).