From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E8FDF332614 for ; Tue, 2 Jun 2026 09:30:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780392657; cv=none; b=gPQtX374UOUptbjK32DOO9Zb8j28dPMj14a4sjzff+MTdXf3J0WVCXlKh9pC02h5ktzbUjqLEPrxgPFAOP8pQnLjR0MDYSyBjs+k96Fm816JxWJFOHcTP0AuWClED2WCHdYWj6ba4YQl28YDxutDQH5vqJhnccv348nKYtaypIo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780392657; c=relaxed/simple; bh=wD3aXT/U7yxRGfk9pu7Je2gODEPjqszDaYTGzz18mkQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=W2QJj/ppZp78vHLyziH/Z1W9Q9GqiaT6oB2aZLVG7tV7UgYQIbDLofBG5ObGsnSX4AsrLYpM23dHLmDjzxWPtO+eYQCcF6ndvxw0/4zYFcY78Q6/U4DvZ9li5ZtZewX5iLG1p4we39iWr55IrknFR94X88EVwbBKJ/ZffGoGg54= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Cc4WT3Dq; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Cc4WT3Dq" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 68E621F00893; Tue, 2 Jun 2026 09:30:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780392651; bh=RhX66fkdtzWkODIz9nI8l9eW5veNrNS+XM6ojvk1z5Q=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Cc4WT3Dq2NWosuhrUMUzUiXO+MBDXbLJeBWfS44QcT6hNBAqlNBQ53D/0pBWUbdPK maaSukjU3EiZDDqMHGn394m+2Amx56dgaDHmDNYMT9osAtFCvRsgJvBEM5TvTA2bKG jfgiOuLdK2d8PK+iI+lAjraUFb09GyVb0XBQj10CCiMW0zDhtNzYg62h34KPrxx8T7 yDs9krGOYgE8MIdjMLI200MM2R+XVmu/telXZZrWbvOsiECfUj0nkgGC/fs5lNu2AY cMasxdWResCX8YUZ1F3BW3DJfBzB5iiloyTY9JXRNFy7yJprbmv0+HR10V4w5USh15 XuDWhFOBDalVQ== Message-ID: <6dd5f74f-b454-46a0-b2f7-ba906792f5ba@kernel.org> Date: Tue, 2 Jun 2026 11:30:47 +0200 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] mm: page_isolation: Avoid hugepage scan step underflow To: Kaitao Cheng , Andrew Morton Cc: vbabka@kernel.org, surenb@google.com, mhocko@suse.com, jackmanb@google.com, hannes@cmpxchg.org, ziy@nvidia.com, liushixin2@huawei.com, osalvador@suse.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Kaitao Cheng , Muchun Song References: <20260519121646.40833-1-kaitao.cheng@linux.dev> <20260519105424.e422131a0ffb49e19ed1aebe@linux-foundation.org> <4bfe16a2-3bb9-44e2-8b3b-8c77857a4944@kernel.org> <88aa1f75-1cdd-499e-b352-0cd6c7f0d079@linux.dev> 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 6/2/26 09:08, Kaitao Cheng wrote: > 在 2026/6/2 00:21, David Hildenbrand (Arm) 写道: >> On 5/22/26 11:35, Kaitao Cheng wrote: >>> 在 2026/5/20 01:54, Andrew Morton 写道: >>> >>> The direct user-visible effect is not memory corruption, but degraded forward >>> progress in the page isolation / contiguous allocation path. >>> >>> If the race makes folio_nr_pages() return 1 while the current page is still >>> treated as a tail page of the old HugeTLB folio, the computed step can >>> underflow: >>> >>> step = folio_nr_pages(folio) - folio_page_idx(folio, page); >>> >>> The caller then does: >>> >>> start_pfn += step; >>> >>> With unsigned arithmetic this can wrap and move start_pfn backwards, typically >>> near the beginning of the old hugepage range rather than advancing past it. In >>> many cases this only means rescanning part of the same hugepage range, so the >>> effect may be limited to extra scanning work. >>> >>> However, it still violates the scanner's forward-progress assumption: step >>> is expected to advance start_pfn. If the same transient state is observed >>> repeatedly, the scanner can keep revisiting the same PFNs, causing excessive >>> latency and, in the worst case, an apparent stall in operations that rely on >>> page isolation or contiguous allocation. >>> >>> >>> Here is another point raised by AI, the old code also used folio_test_lru() >>> on a folio pointer obtained without holding a reference. If the folio is >>> freed and the old head page is reused or observed as a tail page of another >>> compound page, folio_test_lru() can reach const_folio_flags(), which asserts >>> that the passed folio is not a tail page. On DEBUG_VM kernels, that can >>> trigger a VM_BUG_ON_PGFLAGS() crash. >>> >>> >>> Following David Hildenbrand's suggestion, I made some changes as shown below. >>> I'm not sure whether there are still any other issues. >>> >>> --- a/mm/page_isolation.c >>> +++ b/mm/page_isolation.c >>> @@ -41,8 +41,14 @@ bool page_is_unmovable(struct zone *zone, struct page *page, >>> * We need not scan over tail pages because we don't >>> * handle each tail page individually in migration. >>> */ >>> - if (PageHuge(page) || PageCompound(page)) { >>> + if (PageCompound(page)) { >>> struct folio *folio = page_folio(page); >>> + unsigned long nr_pages, pfn; >>> + unsigned int order; >>> + >>> + order = compound_order(&folio->page); >>> + if (order > MAX_FOLIO_ORDER) >>> + return true; >> >> Would we also have to care about non-order-of-2? > > There should not be any non-order-of-2 compound pages here. Neither the > buddy allocator's allocation model nor the folio_size() / size_to_hstate() > implementation supports non-order-of-two compound pages. Ah, sorry, in scan_movable_pages() we use folio_nr_pages(), and that can give you on races a non-order-of-2. So yeah, clearly not required here. > >>> >>> if (folio_test_hugetlb(folio)) { >>> struct hstate *h; >>> @@ -54,15 +60,16 @@ bool page_is_unmovable(struct zone *zone, struct page *page, >>> * The huge page may be freed so can not >>> * use folio_hstate() directly. >>> */ >>> - h = size_to_hstate(folio_size(folio)); >>> - if (h && !hugepage_migration_supported(h)) >>> + h = size_to_hstate(PAGE_SIZE << order); >>> + if (!h || !hugepage_migration_supported(h)) >>> return true; >>> - >>> - } else if (!folio_test_lru(folio)) { >>> + } else if (!PageLRU(page)) { >> >> Hm, is that required because we could VM_BUG_ON? > > Yes, that is one of the reasons for the change. > > page_is_unmovable() does not hold a reference on the folio, and the folio > can be freed and reused concurrently while this scanner is running. In that > case, the previously obtained folio pointer may no longer refer to a valid > folio head. If the original head page is reused as a tail page of another > compound page, folio_test_lru(folio) can hit the folio flag checks and > potentially trigger a VM_BUG_ON_PGFLAGS(). Okay, spell that out in the patch description. -- Cheers, David