From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793783616; b=W92pZAKuig0L2syLEjrTz53SZPyR21Wod/3s8LPlCsPHWUgBhf0+a3N2E9ndRz9lRkdz WzbdrGVmg41dJKV09a5uq95Bl6c2NKuxQcnEOQVXQ9dcsOySWBPPEhTAkFRekvEzsyY/W +yTqXyTcQEwix6tXg1vmtl9tpAWynuszQC27EMiOidOPkjtbmKZx/L3RyDTy9cAXGzDfU BA3B6GiZBUGnQM9NSqPUbOwXT12SnHzpvbkmJ7KkpJVEyyKjzq3n4Ev5QlBrXetRGgiWR GGBtIkLN6GuLLyJTExuBfP7GRE8ji+dus4ha69W5LT8T+lO7Tprg3yYceRH1XL9Rm+Q== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793783616; h=Received:DKIM-Signature:Message-ID:Date:MIME-Version:User-Agent: Subject:To:Cc:References:From:Content-Language:Autocrypt:In-Reply-To: Content-Type:Content-Transfer-Encoding; bh=LoVFgUlolmEAFzVK6w+JrxJ56f3iy7qOBFWwy6NZkbw=; b=nkKdIpWXqlbMXOdAlLomuyyxJL2pufqJIjxOxXRRMp0KCpG4Dzs4co5lCdOHmJyFrlfQ cB/pflAvpdQLsKb/2Dn+pLLz25AxkTiHpiaR8htzalcoUDCjrgOAeZmRQKYAudhcDMgzY mD3q82YiYckbAA0d7yUtRJUC4dcweMgDVwhNCIeDcXbrP1Pe4bJYeXrTZf8EUz87hyTsh kz4s8AvwJhtUySN0VbsUPxSmPIIQxXSmjf/uzEcOIJX2QsdH4aLdRo1jITRbP8bgF+4qM jv9kyDr1NrclEMgZvms2hP0SDnmRZf3JiMqM31JFCvlg0IMbO9meAXyRG6RtBA2N8Tg== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=dRpMfbpZ; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 08749606A31B; Wed, 4 Nov 2026 09:13:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=dRpMfbpZ Received: by smtp.kernel.org (Postfix) with ESMTPSA id 943551F000C1; Wed, 4 Nov 2026 09:13:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793783615; bh=LoVFgUlolmEAFzVK6w+JrxJ56f3iy7qOBFWwy6NZkbw=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=dRpMfbpZeYPKVhajyjXkLtGaqpGPvrZkk4yFqG2ypBe4K+tDcX4rTCHEt04jW5/KG 2F+gxMEZuG9fPh97FKiWfCEdg8Jkd9hbf+LLNrRXCcKFvn8ZuFpn4P+uJKcQMwTJ09 rh79VO2vtKtrO/B+TP5cbe5rQcohMc86qrFv09zuy8OQLIMUXYdZHWE6CAu6WuPmYE xfKiLlcRwOeZxwMqBPFoAG31ac8gFp6BGwX8n+2lfkFQTHQixGQd2zGd4yCTJ0vdqK iCa7zTYm1z7zGE+0f4dAUL13T6KfHHlugQYFxBLi62Qg3c5Sl/7BCdxw3EpxOPHEG6 HvH2dL3dmOm7w== Message-ID: <979fb046-32dc-4af8-b6a3-177c78f1e557@kernel.org> Date: Wed, 4 Nov 2026 10:13:15 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 18/47] mm/khugepaged: check if PTE table is empty in try_collapse_pte_mapped_thp() To: Baolin Wang , Andrew Morton , Johannes Weiner , Michal Hocko , Qi Zheng , Shakeel Butt , Lorenzo Stoakes , Kairui Song , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Baoquan He , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Kees Cook , Peter Zijlstra , Jann Horn , Pedro Falcato , Hugh Dickins , Pasha Tatashin , Pratyush Yadav , "Matthew Wilcox (Oracle)" , Jan Kara , Zi Yan , Lance Yang , Rik van Riel , Harry Yoo , Alistair Popple , Nico Pache , Ryan Roberts , Dev Jain , Usama Arif , Kiryl Shutsemau , Matthew Brost , John Hubbard , Peter Xu , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Oscar Salvador , Minchan Kim , Muchun Song , Xu Xin , Chengming Zhou , Chris Li , Kemeng Shi , Nhat Pham , Youngjun Park , Gregory Price , Joshua Hahn , Rakie Kim , Ying Huang , Jason Gunthorpe Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, linux-cxl@vger.kernel.org, Muhammad Usama Anjum References: <20261031-fixes_core-v3-0-d6437171ecc3@kernel.org> <20261031-fixes_core-v3-18-d6437171ecc3@kernel.org> <62268831-f6f3-4666-a32f-a77546b95432@linux.alibaba.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 11/2/26 09:20, Baolin Wang wrote: > > > On 10/31/26 5:39 PM, David Hildenbrand (Arm) wrote: >> The existing code is rather horrible and already resulted in bugs in the >> past: we special-case UFFD inserting anonymous folios by grabbing the PMD >> lock early, but ignore any other markers (guard, poison through uffd) that >> could have been inserted into the page table in the meantime, effectively >> dropping them. >> >> Let's just unify and simplify the locking: try to grab the PMD table >> lock and if that fails, flush the TLB, drop the PTE lock, take PT locks in >> the right order and check if the page table is still empty. >> >> We still perform the heavy lifting (zapping the folio ptes) under PTE table >> lock only. >> >> Include for now, as x86's flush_tlb_range() macro >> depends on hugetlb helpers. This dependency needs a separate cleanup. >> >> Both issues this fixes are rather corner cases. Let's not CC stable for >> now. >> >> Fixes: f807123d578d ("mm: allow guard regions in file-backed and read-only >> mappings") >> Fixes: 1043173eb5eb ("mm/khugepaged: collapse_pte_mapped_thp() with >> mmap_read_lock()") >> Tested-by: Gregory Price (Meta) >> Reviewed-by: Gregory Price (Meta) >> Tested-by: Muhammad Usama Anjum >> Signed-off-by: David Hildenbrand (Arm) >> --- >>   mm/khugepaged.c | 64 ++++++++++++++++++++++++++++----------------------------- >>   1 file changed, 31 insertions(+), 33 deletions(-) >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index ce794d9760a87..ad07c8caa3bd9 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -10,6 +10,7 @@ >>   #include >>   #include >>   #include >> +#include >>   #include >>   #include >>   #include >> @@ -1832,6 +1833,17 @@ static enum scan_result set_huge_pmd(struct >> vm_area_struct *vma, unsigned long a >>       return SCAN_SUCCEED; >>   } >>   +static bool pte_table_empty(pte_t *pte) >> +{ >> +    int i; >> + >> +    for (i = 0; i < HPAGE_PMD_NR; i++, pte++) { > > Using PTRS_PER_PTE to iterate PTEs seems more straightforward? Otherwise LGTM. As I pulled this patch out from the stable fixes, I fixed it up. I realized that this helper should likely move to some header later and be reused elsewehere (zap_pte_table_if_empty), but that's something for a later cleanup. -- Cheers, David