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 C2DEA3932FA for ; Wed, 4 Mar 2026 09:22:49 +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=1772616170; cv=none; b=XUlGPYHsHAl64VtuawdhEjIrW2htesRD5/ZiAKrnPnxT0PX9DQMjY4K7SuIYG54UNJ+6SSiZFAsaDpHHMFJNgsjjnIgonaRipexVPVuzwZWCkRWCNPYiv1Lji9xxahyF9p+DouExRrnQpCbMqWECisma5in9BmIUwIedXVg04aY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772616170; c=relaxed/simple; bh=XswYbNMQzamtRva8tDTkOZtyOlflbjaP5OIV21nZX8g=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Qn83kuk80GNnVdw/NWXlqV0y2s8eRJDs7lXwGBRGGJomZned6vg9/wmUTuDBCTXLnnZKp4TP0KUp5NrdiTnn8Udt0lLaMFP06CN7ior56VraU2aP2I8qL4ApOXzU7euFuYYyYA4sbik0wV+4jBTU+94lN6pvcwzP+o230A40XZo= 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 019F3339; Wed, 4 Mar 2026 01:22:43 -0800 (PST) Received: from [10.163.173.252] (unknown [10.163.173.252]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id EA8673F694; Wed, 4 Mar 2026 01:22:43 -0800 (PST) Message-ID: <346a421b-5b8e-41b7-b92e-a5a679030f1f@arm.com> Date: Wed, 4 Mar 2026 14:52:40 +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 v2] khugepaged: remove redundant index check for pmd-folios To: Dev Jain , akpm@linux-foundation.org, david@kernel.org, lorenzo.stoakes@oracle.com Cc: 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: <20260227143501.1488110-1-dev.jain@arm.com> Content-Language: en-US From: Anshuman Khandual In-Reply-To: <20260227143501.1488110-1-dev.jain@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 27/02/26 8:05 PM, 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, remove this check. > While at it, simplify the comments. > > Signed-off-by: Dev Jain Reviewed-by: Anshuman Khandual > --- > v1->v2: > - Remove the check instead of converting to VM_WARN_ON > - While at it, simplify the comments > > Based on mm-new (8982358e1c87). > > mm/khugepaged.c | 14 ++++---------- > 1 file changed, 4 insertions(+), 10 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 5f668c1dd0fe4..b7b4680d27ab1 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -2015,9 +2015,7 @@ 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) { > - /* Maybe PMD-mapped */ > + if (folio_order(folio) == HPAGE_PMD_ORDER) { > result = SCAN_PTE_MAPPED_HUGEPAGE; > goto out_unlock; > } > @@ -2345,15 +2343,11 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, > continue; > } > > - if (folio_order(folio) == HPAGE_PMD_ORDER && > - folio->index == start) { > - /* Maybe PMD-mapped */ > + if (folio_order(folio) == HPAGE_PMD_ORDER) { > result = SCAN_PTE_MAPPED_HUGEPAGE; > /* > - * For SCAN_PTE_MAPPED_HUGEPAGE, further processing > - * by the caller won't touch the page cache, and so > - * it's safe to skip LRU and refcount checks before > - * returning. > + * PMD-sized THP implies that we can only try > + * retracting the PTE table. > */ > folio_put(folio); > break;