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b=Cq9o+qdKB669g0wfM0rojSPXacwLxsqfRMHKYCuNnN4YUTImI/Y0eqN6flYTL+L4U bU74vNyexoGx61OsHBjsjmJDH6fNHMLsnw0W/rNjKb2Rzp56XvlpZ14OAlB/Z3m62W 6LeVcrFrf1IepTPiAQmEGaU66PY3knjLyIAfet6pbi0YrktGqp94k85g13m7ob5HSz yKDdkGNA7wAwWqqnTuQs1S4pzXDTJrjF1L4KwmJ41sPjDJq9dzYq+8BtU6xGdcsJyO Jjxlur43LTFIyCyRN/SGCCZpQfwBtxAJ21QU1r76Uv5TCOvF34TZLXZ0lZQlj8B6qk Gb0lMFlTxEOtg== Message-ID: <24b7829b-aaf0-42ac-b91c-916754f95046@kernel.org> Date: Tue, 24 Feb 2026 11:51:23 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] khugepaged: convert redundant check to WARN_ON To: Dev Jain , Andrew Morton Cc: 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> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: u1ktx33ot57bi6dsuoahc9medy9zz5my X-Rspamd-Server: rspam11 X-Rspam-User: X-Rspamd-Queue-Id: CEA7820004 X-HE-Tag: 1771930290-866401 X-HE-Meta: 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 I4tmCZ6K 4ZLKCcq79iIvTiVE9UN4+qm5SsWJxDBVTB3AuMsN1VyTeX0nwl+sPJV+vWFwyB0uqvbnjdZHe2cEs3sM7ZbaujdpVZ53o2FdegiJ6sAeAlOUF65+OOAveAJMPW8BOIE22tKucByGzkRq3On1+VXlWxmFOYTbnexKKkYGIT/0wVhsqAajsJNTQ6nG+vtibGoS4B4e8Cn7yCsiNO+pQtPX0f581RK7q0oVtlFZXChBt6phkWb8X2k/E0i4VLBVe9OX8DftwEK1xXEV/a+0Rdim2fT6WJhLCt0Xq4vEK76muOKdDPeeWpUGasboaYVBAOGId19Ink96sSwjNQ+05vC1XjhmqwPxZtkUgZOOyOiBxXJPpoD0= 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 2/23/26 05:40, Dev Jain wrote: > > 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 :) I'd say, remove it directly. Even if "folio->index != start", both functions should do the right thing. While at it, we could heavily simplify the comments to "If there is already a PMD-sized THP, we can only try collapsing the PTE table". Or sth, like that. -- Cheers, David