From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id DA214F46113 for ; Mon, 23 Mar 2026 13:40:25 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 3FD4B6B0005; Mon, 23 Mar 2026 09:40:25 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 3D4BA6B0088; Mon, 23 Mar 2026 09:40:25 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 2EA996B0089; Mon, 23 Mar 2026 09:40:25 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id 1F5C36B0005 for ; Mon, 23 Mar 2026 09:40:25 -0400 (EDT) Received: from smtpin05.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay01.hostedemail.com (Postfix) with ESMTP id C2B921ED62 for ; Mon, 23 Mar 2026 13:40:24 +0000 (UTC) X-FDA: 84577437168.05.18A5D66 Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf14.hostedemail.com (Postfix) with ESMTP id 0D699100003 for ; Mon, 23 Mar 2026 13:40:22 +0000 (UTC) Authentication-Results: imf14.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=ezodwAAY; spf=pass (imf14.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1774273223; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=aWyyESyH59EW5awXlYTiFlcAOR7WM0zfDF5m7dxaQlA=; b=jpZvYXFM9W4UeRxYL9ecbl2aG4s5yU2HA2rtKiHN5aoc6HZQdJMFIxHU0JK7N4Pb8fXACr oGOw8hyY0uWZsufuB+Z19g/f8TNXor4QhcFKqDy85gIplpd0sTHScWoEQMuGgg1EO26BYO kqfGLnFwc03TPRWJEB2AbHn/iWzbmxc= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1774273223; a=rsa-sha256; cv=none; b=Vmhx3vF0WfRXNX/xopYhr9J0NO+AXidWTgL3vlRUQHHOQw5qrJiRm6AI9NqV1239FT9v4m VbmX+1o0Wd8SV8dwpBWW5t6R+2VmyF9DUcqq0OIyBN9/v4Beanzg68nn+3wvBAszSHi4pI LHtIXzhFM5l56VkzS5skQ/eZQ5OjoXE= ARC-Authentication-Results: i=1; imf14.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=ezodwAAY; spf=pass (imf14.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id C21954333A; Mon, 23 Mar 2026 13:40:21 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id DBE1DC2BC9E; Mon, 23 Mar 2026 13:40:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774273221; bh=+vl328zCZ3IIWDg4kXdCU0JqpKru3mTI0xRcjoscBVM=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=ezodwAAYaGvaJuUgQ96ue3bXKBLH00W4fabxcJbDe23Ewjtn0k0Eg7+pc9MgnAQkx b8o1laBdA+mpl8l/nBYulFIsOA2qDeQYWjuqlvKv7N4oPYdLc7T0QOtjSPzdL/yt3C 444zlf3hSnWneG7NX3X1bvmFgBuVh1GslF0GhWAo2MHllphy1APi6H5j1Ydhu4ETXw 3+0+7VO/M0dKJx4YPEQ6MgDYXnvaoDSX2efODknd5T4qpzgE5jwe+4csr6ba6QBW1g HrlbHYre4V3N42RmIusaCwaB3cnQ4HyUBr3THAEbkihqON6pgNSwMACFqdIKSO7vf7 uhqCaRakmgQ7A== Message-ID: <4caebf3b-8924-406f-9b92-909cf86ef669@kernel.org> Date: Mon, 23 Mar 2026 14:40:16 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 01/15] mm/memory_hotplug: fix possible race in scan_movable_pages() To: "Lorenzo Stoakes (Oracle)" Cc: linux-kernel@vger.kernel.org, Andrew Morton , Oscar Salvador , Axel Rasmussen , Yuanchu Xie , Wei Xu , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Sidhartha Kumar , linux-mm@kvack.org, linux-cxl@vger.kernel.org, linux-riscv@lists.infradead.org References: <20260320-sparsemem_cleanups-v2-0-096addc8800d@kernel.org> <20260320-sparsemem_cleanups-v2-1-096addc8800d@kernel.org> <5b489c40-bd89-4dff-9669-560b1b28ec69@lucifer.local> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <5b489c40-bd89-4dff-9669-560b1b28ec69@lucifer.local> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: jxtgbuqrr8hfweuyntwtrmzx3aeb4p1m X-Rspam-User: X-Rspamd-Queue-Id: 0D699100003 X-Rspamd-Server: rspam12 X-HE-Tag: 1774273222-658785 X-HE-Meta: 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 S8nXogbe +4CSPBF5CNceIAjI= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/23/26 14:26, Lorenzo Stoakes (Oracle) wrote: > On Fri, Mar 20, 2026 at 11:13:33PM +0100, David Hildenbrand (Arm) wrote: >> If a hugetlb folio gets freed while we are in scan_movable_pages(), >> folio_nr_pages() could return 0, resulting in or'ing "0 - 1 = -1" >> to the PFN, resulting in PFN = -1. We're not holding any locks or >> references that would prevent that. >> >> for_each_valid_pfn() would then search for the next valid PFN, and could >> return a PFN that is outside of the range of the original requested >> range. do_migrate_page() would then try to migrate quite a big range, >> which is certainly undesirable. >> >> To fix it, simply test for valid folio_nr_pages() values. While at it, >> as PageHuge() really just does a page_folio() internally, we can just >> use folio_test_hugetlb() on the folio directly. >> >> scan_movable_pages() is expected to be fast, and we try to avoid taking >> locks or grabbing references. We cannot use folio_try_get() as that does >> not work for free hugetlb folios. We could grab the hugetlb_lock, but >> that just adds complexity. >> >> The race is unlikely to trigger in practice, so we won't be CCing >> stable. >> >> Fixes: 16540dae959d ("mm/hugetlb: mm/memory_hotplug: use a folio in scan_movable_pages()") >> Signed-off-by: David Hildenbrand (Arm) > > Logic looks right to me, though some nits below. With those accounted for: > > Reviewed-by: Lorenzo Stoakes (Oracle) > >> --- >> mm/memory_hotplug.c | 11 ++++++++--- >> 1 file changed, 8 insertions(+), 3 deletions(-) >> >> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c >> index 86d3faf50453..969cd7ddf68f 100644 >> --- a/mm/memory_hotplug.c >> +++ b/mm/memory_hotplug.c >> @@ -1747,6 +1747,7 @@ static int scan_movable_pages(unsigned long start, unsigned long end, >> unsigned long pfn; >> >> for_each_valid_pfn(pfn, start, end) { >> + unsigned long nr_pages; >> struct page *page; >> struct folio *folio; >> >> @@ -1763,9 +1764,9 @@ static int scan_movable_pages(unsigned long start, unsigned long end, >> if (PageOffline(page) && page_count(page)) >> return -EBUSY; >> >> - if (!PageHuge(page)) > > Yeah interesting to see this is folio_test_hugetlb(page_folio(page)) :)) > > So this is a nice change for sure. > >> - continue; >> folio = page_folio(page); >> + if (!folio_test_hugetlb(folio)) >> + continue; >> /* >> * This test is racy as we hold no reference or lock. The >> * hugetlb page could have been free'ed and head is no longer >> @@ -1775,7 +1776,11 @@ static int scan_movable_pages(unsigned long start, unsigned long end, >> */ >> if (folio_test_hugetlb_migratable(folio)) >> goto found; >> - pfn |= folio_nr_pages(folio) - 1; >> + nr_pages = folio_nr_pages(folio); >> + if (unlikely(nr_pages < 1 || nr_pages > MAX_FOLIO_NR_PAGES || > > NIT: since nr_pages is an unsigned long, would this be better as !nr_pages || ...? It's easier on the brain when spotting that only a given range is allowed, without having to remember the exact type of the variable :) So I guess it doesn't really make a difference in the end. > >> + !is_power_of_2(nr_pages))) > > Could the latter two conditions ever really happen? I guess some weird tearing > or something maybe? Yes, or when the fields gets reused for something else. > > It would also be nice to maybe separate this out as is_valid_nr_pages() or > something, but then again, I suppose given this is a rare case of us > checking this under circumstances where the value might not be valid, maybe > not worth it. I had the same thought. But this code is way too special regarding raciness that I hope nobody else will really require this ... and if they do, they might be doing something wrong :) -- Cheers, David