From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 361A63B27C4 for ; Wed, 1 Apr 2026 09:44:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775036673; cv=none; b=BQyzPgp2522hDU/CWjyJizKoG51VraIk12Vm3cmBS4mXalZoLWgIBvmJXDhe5YNGIrAKXn9AIHsp+7i9qDmoQCwr4yeqoddijMvl44KNwWjWyYIrwwUC2MPkEXZVbcS0oM4Qt0hehSGZ0duBsxKBEiMpyVv80+7VSWJYzTx9ooI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775036673; c=relaxed/simple; bh=2oOw8zs3xzGzoF5YHSLqg3ICLDD+b5wmvRMAkOMujWQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=lavGkroqN7lHWL10t8bj5l66VXpQWBT11tYzM86mn7HcjnbHFGdZrP4e00+y0rsV65uNYAXejkzQ017K9lp2dFSqSqAgESO5FIuV3j/ry8N3p5hBVI5Ksqg/3AmMgqcIcsMczjfRF2lVRSvO1Ac1dvF/CgJMYmlYGoIsVSKAddA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=U37bawTX; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="U37bawTX" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 23A4FC4CEF7; Wed, 1 Apr 2026 09:44:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775036672; bh=2oOw8zs3xzGzoF5YHSLqg3ICLDD+b5wmvRMAkOMujWQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=U37bawTX8UJU2iy9qXdm6B9yw7N6+y8LKL2hpL/2a4wRUqRCCff7JpdXiBT6ZGDi3 fKcELR9dEua8h5+ndQSAVcAK0AjTAfqNxmsMs/+bag8AvPg1gFoX5NsPsR0iVFJ2Se IgJcdtji09oWqQ5EVQQKpRYLkBQoj4yLK+udxeQLf2hZW5Bu/Gx6uNdjpBhc0OuvwQ XQ5asT7Bv9eqLrzo+zee2wfKW8Jj7koruuSe9X+oPbXCM8Ghabee49lhDw2rey05+Y FAzX9Re1KvMMbZygqHSNLEgVg2VFPI1Ln1gaawgTf0o75xgLtazaplF+YIO7odWJiG q4IdO76V8w38A== Message-ID: Date: Wed, 1 Apr 2026 11:44:27 +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/migrate: fix stale partially_mapped arg to deferred_split_folio() To: Deepanshu Kartikey , akpm@linux-foundation.org, ziy@nvidia.com, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com Cc: usama.arif@linux.dev, richard.weiyang@gmail.com, sj@kernel.org, hannes@cmpxchg.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com References: <20260401084116.22219-1-kartikey406@gmail.com> 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: <20260401084116.22219-1-kartikey406@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/1/26 10:41, Deepanshu Kartikey wrote: > In migrate_folio_move(), src_partially_mapped is sampled from the source > folio before move_to_new_folio() is called: > > if (folio_order(src) > 1 && > !data_race(list_empty(&src->_deferred_list))) { > src_deferred_split = true; > src_partially_mapped = folio_test_partially_mapped(src); > } > > A concurrent thread can unmap pages from the source folio between this > read and the actual migration, making the sampled value stale. Indeed. We could see "partially_mapped" getting set concurrently. > > After move_to_new_folio() succeeds, __folio_migrate_mapping() has already > copied all folio flags including PG_partially_mapped from src to dst. > Passing the stale src_partially_mapped=false to deferred_split_folio(dst) > while dst already has PG_partially_mapped=true triggers the invariant > check in deferred_split_folio(): > > VM_WARN_ON_FOLIO(folio_test_partially_mapped(folio), folio) > > at mm/huge_memory.c:4371, because the argument contradicts the flag > already set on the folio. > > Fix this by removing the src_partially_mapped variable entirely and > reading PG_partially_mapped directly from dst after move_to_new_folio() > completes, where it is authoritative and race-free. > > Fixes: 8a8ca142a488 ("mm: migrate: requeue destination folio on deferred split queue") > Reported-by: syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com > Closes: https://syzkaller.appspot.com/bug?extid=a7067a757858ac8eb085 > Tested-by: syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com > Signed-off-by: Deepanshu Kartikey > --- > mm/migrate.c | 10 +++++----- > 1 file changed, 5 insertions(+), 5 deletions(-) > > diff --git a/mm/migrate.c b/mm/migrate.c > index 05cb408846f2..11236779e910 100644 > --- a/mm/migrate.c > +++ b/mm/migrate.c > @@ -1361,7 +1361,6 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private, > int old_page_state = 0; > struct anon_vma *anon_vma = NULL; > bool src_deferred_split = false; > - bool src_partially_mapped = false; > struct list_head *prev; > > __migrate_folio_extract(dst, &old_page_state, &anon_vma); > @@ -1378,7 +1377,6 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private, > if (folio_order(src) > 1 && > !data_race(list_empty(&src->_deferred_list))) { > src_deferred_split = true; > - src_partially_mapped = folio_test_partially_mapped(src); > } > > rc = move_to_new_folio(dst, src, mode); > @@ -1404,11 +1402,13 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private, > /* > * Requeue the destination folio on the deferred split queue if > * the source was on the queue. The source is unqueued in > - * __folio_migrate_mapping(), so we recorded the state from > - * before move_to_new_folio(). > + * __folio_migrate_mapping(). Read PG_partially_mapped directly from > + * dst: move_to_new_folio() copies all flags from src to dst, so dst > + * now holds the correct authoritative state. Where are we copying PG_partially_mapped exactly? I don't see that happening in folio_migrate_flags(). Also, I wonder what happens if the folio gets partially mapped after already installing migration entries (zapping migration entries). This is all extremely shaky. -- Cheers, David