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Tue, 24 Feb 2026 16:21:40 +0800 Message-ID: Date: Tue, 24 Feb 2026 16:21:39 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] khugepaged: convert redundant check to WARN_ON To: Dev Jain , akpm@linux-foundation.org, david@kernel.org, lorenzo.stoakes@oracle.com Cc: ziy@nvidia.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> From: Baolin Wang In-Reply-To: <20260219054827.4111334-1-dev.jain@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Stat-Signature: coxy3aegbnrmf4pph7u3f7m3f8xcgo56 X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 6B56580016 X-HE-Tag: 1771921305-835664 X-HE-Meta: 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 X7n6fLI3 ch5Od9UYKaikveH2hzWgxsCzBR9kYc7O9beTOMqnvXZ0z/YTazBffDRgffp2iKeXsn5qMreHJlmXTm4BRodxSqZSSL+laSjpljArwnUQ5NMkL1bx5XBvyjAqW65i6eAAgK1sX7az23s1UWA8cFPVyIeSJUsMFAKR8LRKVJt38uuMHysOyG88rMhYAu/Tot0yvGjuAgWi+a4oX58cMd5/Ooq0joJqEW/ULOEncdP48AItFmA53t6jLLn5nyPVmofCg7N6g5sbCsniI8Yg9GoEav9L45EO3+M8BYCJhpBuXLgULcxmdvJpe8UWVNdxnJ67QtSPn0tMWwYCl8zFp3Vnh9e3Dt9U8p58qL1FtyHDh5I9cdQ0ShAhSmG8EEUqVv6GTacJmLjzknCrtnNP4K9ZXQFsVNcsftyhMR1y/RI+JtOYpDgnZfs2/UgL5SA== 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 2/19/26 1:48 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, convert this to a VM_WARN_ON. > > Signed-off-by: Dev Jain > --- Make sense to me. Personally, I’d like to keep this VM_WARN_ON() to catch unexpected behavior. Reviewed-by: Baolin Wang > Based on mm-unstable (d9982f38eb6e). mm-selftests pass. > > mm/khugepaged.c | 10 ++++++---- > 1 file changed, 6 insertions(+), 4 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index fa1e57fd2c469..f27cbb4d1f62c 100644 > --- 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); > + > /* Maybe PMD-mapped */ > result = SCAN_PTE_MAPPED_HUGEPAGE; > goto out_unlock; > @@ -2329,8 +2330,9 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned > continue; > } > > - if (folio_order(folio) == HPAGE_PMD_ORDER && > - folio->index == start) { > + if (folio_order(folio) == HPAGE_PMD_ORDER) { > + VM_WARN_ON(folio->index != start); > + > /* Maybe PMD-mapped */ > result = SCAN_PTE_MAPPED_HUGEPAGE; > /*