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 EDF342DECBA for ; Mon, 1 Jun 2026 16:21:17 +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=1780330879; cv=none; b=EaM+8F1LEXiFQ/xC4UDRLMNgkpnq/OmdeMFHDk3b0wY1hrLqm+il/Ywzd5OQFwdxj2u7SiwXeu5hVc/Ld75t8GqKeF1gkIqRgo4p9nfy9qGwwI9efS+E9TUsaY+t9+wAhau5t4ahdQvpLOiaw9CBlKbu2+548a7KnCuZ8OOd3Qg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780330879; c=relaxed/simple; bh=Qm2wGXo73nDbVmTPGZ5FMT2Qvfd0vR/qdb0e4G7IVKg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=GTTWt2mRed+rRe0xUK6e/mm8E6OYKoBsSy4U/Wss974aZZYaqBllK1q+tqUtsJdvksMTOPrBIoTj1vywBb+azFGD4RLn6u3uwkpqyQ38j+i6hG5mhhx60hj/OhH0cBmoQzYt1nXWPbvikxVmvJHPuBV27uQrCAtxnbNarRbdbJI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=kKxrUQgd; 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="kKxrUQgd" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E14591F00893; Mon, 1 Jun 2026 16:21:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780330877; bh=UoJTMcm6ZaNDfWUE5LlijECfpdOWKQWYDYlCnlwE1HI=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=kKxrUQgdbCyqXXD74CHwzYgpuVJrv4CK0/FjjDUYIIGwJRF+Iy6ZZZThDs7Tce/13 JH77VQPXaQ1C1ujjS5cYSM9D8Qf46gH/6/fHprkFELNguGv9ULTnoj2AI59TCTLTKZ gPWwN3GuXjZpfA1sxGC7Yj2GRhiStF+4FAlrp/qgtQDaR02JEejxQ7VsEdsqbrf7NE 9o58Yc91zhvfusXivt3JV3YoAelAb/81xp+Q5gule+z2OcqSP7yoK1HOKHhwUrXNYd iB7IisbYoQIHDy97FmDBZGLRz8jvfjilHAGpiTFSuJ092/14ALiaTNFbxP4MHb1inK Ef+7+RTm5IKDw== Message-ID: <4bfe16a2-3bb9-44e2-8b3b-8c77857a4944@kernel.org> Date: Mon, 1 Jun 2026 18:21:12 +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> 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 5/22/26 11:35, Kaitao Cheng wrote: > 在 2026/5/20 01:54, Andrew Morton 写道: >> On Tue, 19 May 2026 20:16:46 +0800 Kaitao Cheng wrote: >> >>> page_is_unmovable() checks HugeTLB pages without holding hugetlb_lock and >>> without pinning the folio. This is intentional for the pageblock scanning >>> paths, but it means the HugeTLB folio can be freed concurrently after >>> PageHuge() or folio_test_hugetlb() succeeds. >> >> Thanks. What are the userspace-visible runtime effects of this? > > 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. > >>> The existing code avoids folio_hstate() and uses size_to_hstate() because >>> the HugeTLB flag may already have been cleared. However, if >>> size_to_hstate() returns NULL, the code still falls through and computes >>> the scan step from folio_nr_pages(). If the folio has been freed and the >>> head/large state has been cleared, folio_nr_pages() can return 1. When the >>> current page is a tail page, subtracting folio_page_idx() from 1 can >>> underflow and make the scanner skip too far. >>> >>> Treat a NULL hstate as unmovable so the scanner does not try to skip over >>> an unstable HugeTLB folio. Once a valid hstate is found, derive the number >>> of pages from the hstate instead of reading the folio size again. Also >>> validate the page index before computing the step to avoid underflow if the >>> page/folio relationship changed concurrently. >> >> This code sounds rather sketchy, and it sounds like it will remain >> sketchy after the patch. And AI review says "hey, this code is >> sketchy": >> https://sashiko.dev/#/patchset/20260519121646.40833-1-kaitao.cheng@linux.dev > > 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? > > 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? > return true; > } > > - *step = folio_nr_pages(folio) - folio_page_idx(folio, page); > + nr_pages = 1UL << order; > + pfn = page_to_pfn(page); > + *step = (pfn | (nr_pages - 1)) + 1 - pfn; > return false; > } > -- Cheers, David