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 4D74E445ACE for ; Wed, 14 Oct 2026 09:25:25 +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=1791969926; cv=none; b=o6+8zrrZN5ZsiqDiU/uy8TIEtoC3rcNLR4poROlvi/+NTleAwbLqezzvbA4OiLVjSG1b/LSSFosc9h2Vn1s8tEkUzRl7iu7tVuSIDYcFDi1OeQrZtjie8uykYJ9zbLkrlYivjzzSUnPfsI0weqDmKFYG/Y9PyZVYWGrRlzZrbDM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791969926; c=relaxed/simple; bh=VeBFQxGl/oAezwwhR30tJyZj0EmnILZeESfiliEEVW4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=R+2GoemDZrjdeDBPZqddo7s/bOhQCvDwY9Gvyq5kR8ETzpdVn2CVVtn+6U1P27p5HT+6nciGCl5hhOWqI68G7RVBTGLu8C+UJ39WwL8iu7cc/uFqu2QkVBqketbmYcKmOx0Z+5oRsm7CtqSoA9jD3Iq6jPOeFsK103mHMcCOH7o= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=jO5b6GV/; 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="jO5b6GV/" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0744B1F000FF; Wed, 14 Oct 2026 09:25:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791969924; bh=YmX2eNFe+FELApsNl9w69BnQeqx//n+AuznHDFMkOG8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=jO5b6GV/KYqQ7oqIp7paAGm+LoOkutQPFYBgrE51ZQVTT6zBBkawBB0SLo+eAJPx4 VmmtydqIT7Itxb+b+vKFq/O5uCqfeumNnryIn5xE9K9v5NmIvLjyPFNyvcvj2Ys5Zm 9m/Gbb2reBNLzZnFZQlGkSDzJop65XJsQ3BrDMfwyFILa0UlsSlNjeaCZak6vtRbl3 79vHpC+8T7Kyv8fuOVom/jk9P53ogPcgqOCvGsjCJoccr6CipcmqFcgtXyvHB+Q33t H+DU4kO8Jo4kE/KiFivqBaKz3gXj9qwRLwM002ihN3n/2XBXJGYe+lki7YKomxq8PR oZpqeMgjTSKBA== Message-ID: <9f4c7a96-7cbe-431e-afcc-8bf7224e1dfd@kernel.org> Date: Wed, 14 Oct 2026 11:25:18 +0200 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/madvise: atomically clear the young bit on THP MADV_COLD To: Pedro Falcato , Andrew Morton , "Liam R. Howlett" , Lorenzo Stoakes Cc: Vlastimil Babka , Jann Horn , Minchan Kim , Michal Hocko , Johannes Weiner , linux-mm@kvack.org, stable@vger.kernel.org, Samarth Narang References: <20261013191719.95502-1-pfalcato@suse.de> 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: <20261013191719.95502-1-pfalcato@suse.de> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/13/26 21:17, Pedro Falcato wrote: > As-is, madvise_cold_or_pageout_pte_range() discards the hardware young > bit by invalidating the PMD, doing pmd_mkold() and subsequently writing > to the pmd again. However, it does so in a racy manner, looking at a > possibly-stale snapshot. It's entirely possible that a write elsewhere > set the hardware D bit, which this logic now loses. > > Fix it by using pmdp_test_and_clear_young(). This is an atomic > modification of the PMD entry, and thus safe against any possible racing > writes. As a (nice) side-effect, this elides one non-required TLB flush > in pmdp_invalidate(). > > This bug can interact with the swapping system in complicated ways, leading > to data loss. For instance, if a folio is marked lazyfree, but was later > read from, and then MADV_COLD'd while racing a write to that same THP, > dropping the dirty bit may lead to data loss as reclaim/MADV_PAGEOUT drops > the "clean" anon folio: > > T0 T1 > ================================================================ > MADV_FREE(thp) | > folio marked lazyfree | > read(thp) | > PMD marked young by hardware | > | MADV_COLD(thp) > write(thp) | if (pmd_young(pmd)) { > PMD marked dirty by hardware | set(pmdp, pmd_mkold(pmd)); > MADV_PAGEOUT/reclaim | > dirty bit may or may not be | > set | > reclaim/pageout may drop | > the folio | > > Fixes: 9c276cc65a58 ("mm: introduce MADV_COLD") > Cc: stable@vger.kernel.org > Reported-by: Samarth Narang > Closes: https://lore.kernel.org/linux-mm/20261011132227.413217-1-samarth.colleges@gmail.com/ > Signed-off-by: Pedro Falcato > --- > mm/madvise.c | 5 +---- > 1 file changed, 1 insertion(+), 4 deletions(-) > > diff --git a/mm/madvise.c b/mm/madvise.c > index 85ab9bc76e06..19cd3bb4d1b9 100644 > --- a/mm/madvise.c > +++ b/mm/madvise.c > @@ -431,10 +431,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd, > } > > if (!pageout && pmd_young(orig_pmd)) { > - pmdp_invalidate(vma, addr, pmd); > - orig_pmd = pmd_mkold(orig_pmd); > - > - set_pmd_at(mm, addr, pmd, orig_pmd); > + pmdp_test_and_clear_young(vma, addr, pmd); > tlb_remove_pmd_tlb_entry(tlb, pmd, addr); Ah, we even already have that arch helper, nice. Let's hope all relevant architectures implement a proper variant :) Acked-by: David Hildenbrand (Arm) -- Cheers, David