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 772762DC334 for ; Tue, 2 Jun 2026 19:05:24 +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=1780427125; cv=none; b=AF3VGFEtqOCpKBkLKdFIq+m4OOF2uzox8yOYreTp2gaPtrN4pJXx+gJu+mlpIX+20wLNMlwB7hYr6Mgqj7T5bUCn2kE9Xl00dtYiHDvre1fwqeWnFyzvYXv2tVpiE6JvIhTl1uSUiHvyzRe6zXoNzs43RB/xPC1flUmKCOmbD9k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780427125; c=relaxed/simple; bh=C+e+j17GlNP1fcxrkSvcB+ZRiLOQoZcFv3O2Fo8Zr5U=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=JWpqmSW1IjpM4lnx+/8RHln8alYoowppazWvu9O+mP9EwzAnhauGLhHH/sUIPPiQ849yPsnBJZk2oJZIt1Kup9h3gsANmOuj+ZIaI38SphniaOYpqkHmrLghE7tKgMX3kPdJIdzIDLqpeK2FYwy8RcxUfWYdp9TRC0ou2FNSF6E= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=I0mNp4fi; 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="I0mNp4fi" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C07161F00893; Tue, 2 Jun 2026 19:05:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780427124; bh=f4ZyRdhtCO1dv7T3Lj/KxS0Pm19iL4/0dY7jaN49Kns=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=I0mNp4fipHMFLsjeiHarIAc/AwZDY9TKHt5YzeQ0/nt867h0ePIjiGzboazBeD7E/ qf3KhcpySwgWtRMQNm5/JpW03pqS6I+yw+4ZiNSlB2tv1R4Ah6SIhNWfWudHg5Fpwo mLJpvrfaDRa+5Obu3VMjc2xdccC3NcKIRkV2x4Yz3S3ZzOakmyhrqVeobC2Cx1C6l+ 6e1zs74Jdov87CNBJlD5NVUBetgD9peYD7SvmH7NEZC7FjaDhNxjCRE76Ru85t7FZd FTcNqVIh2orMCl4iF75xI6ioixzwDNuOfKsDa6RIhPOA56Cf7MeW0O2gxEBuHWjbCq +hwjmm5JiVU+A== Message-ID: <7d199cfd-cf45-4545-b9fd-19b9dbee4b6e@kernel.org> Date: Tue, 2 Jun 2026 21:05:19 +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 v2] mm: page_isolation: avoid unsafe folio reads while scanning compound pages To: Kaitao Cheng , Andrew Morton , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan Cc: Liu Shixin , Oscar Salvador , muchun.song@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Kaitao Cheng References: <20260602130755.38794-1-kaitao.cheng@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: 7bit On 6/2/26 15:07, Kaitao Cheng wrote: > From: Kaitao Cheng > > page_is_unmovable() can inspect compound pages without holding a folio > reference or any lock. The folio can therefore be freed, split or reused > while the scanner is still looking at it. > > The existing HugeTLB handling already avoids folio_hstate() for this > reason, but it still derives the hstate from folio_size() and later > derives the scan step from folio_nr_pages() and folio_page_idx(). > These helpers rely on the folio still being a valid folio head. If > the folio changed concurrently, the scanner can read inconsistent folio > metadata and compute a wrong step. In the worst case, folio_nr_pages() > can return 1 for what used to be a tail page and the subtraction from > folio_page_idx() can underflow. > > There is a similar issue for non-Hugetlb compound pages: folio_test_lru() > expects a valid folio. If the previously observed head page has been > reused as a tail page of another compound page, the folio flag checks > can trigger VM_BUG_ON_PGFLAGS(). > > Read the compound order once with compound_order(), reject obviously > bogus orders, and derive the hstate and scan step from that order > instead of querying folio size information again. Also use PageLRU(page), > which is safe for the page being scanned, instead of folio_test_lru() > on a potentially stale folio pointer. > > Treat an unknown HugeTLB hstate as unmovable so the scanner does not try > to skip over an unstable HugeTLB folio. > > Fixes: a0a9f2180b90 ("mm: page_isolation: avoid calling folio_hstate() without hugetlb_lock") > Signed-off-by: Kaitao Cheng > --- > Changes in v2: > - Avoid unsafe folio metadata reads in the unlocked scanner by deriving > the hstate and scan step from compound_order(). (David Hildenbrand, > Andrew Morton) > - Treat invalid compound orders or unknown HugeTLB hstates as unmovable. > - Use PageLRU(page) instead of folio_test_lru(folio) to avoid folio flag > checks on a stale folio pointer. () > - Update the commit log (David Hildenbrand) > > Link to v1: > https://lore.kernel.org/all/20260519121646.40833-1-kaitao.cheng@linux.dev/ > > --- > mm/page_isolation.c | 19 +++++++++++++------ > 1 file changed, 13 insertions(+), 6 deletions(-) > > diff --git a/mm/page_isolation.c b/mm/page_isolation.c > index 7a9d631945a3..32ce8a7d9df3 100644 > --- 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)) { Nit: you're not spelling that change out in the patch description, but it's alright. > 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; > > 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)) This change might have been better in a separate patch, but it should also be alright (and never really trigger, ever ...). Acked-by: David Hildenbrand (Arm) -- Cheers, David