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 83812241665; Tue, 24 Mar 2026 12:46:25 +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=1774356385; cv=none; b=o1c73NztyPi5SWA2CaUFsuuJSSR6GeSNFzOQwPIuqvXOWrewoLK5WrR8wFoIXFTixVVqHaLCEFoD6D/e4QCcnw/aDl3v4vkxqEm3eVwYoUZneaYbAnkl5v0DHYY3NUk7u7aSvs8tOx0Wyu+dP5Kdgn/ciTzfs1Ain1cYDogzp9k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774356385; c=relaxed/simple; bh=pj0qNOEO5MNCC+dnvNkS1WO0CHnBtfXliGY5/GqJgPY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=MdZYdBNZ/gEwOIxWPLeDDKPUjU1plRnvI/DNhGGKCBxyAzl+rYXN8VkDvFiryIeXDG+DK6Wi3EFcRcuZksTGQo1LXp1dE9nNhbcWOw4aZybfXrrye2J4jXl1CBKyyw36zvdnCxjipLWHRSm8G+SdIj8+AEcBkdo8Gv2fUZlvQSg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ZxGj9ugq; 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="ZxGj9ugq" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 75D74C19424; Tue, 24 Mar 2026 12:46:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774356385; bh=pj0qNOEO5MNCC+dnvNkS1WO0CHnBtfXliGY5/GqJgPY=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=ZxGj9ugqZy4OaUMpJcbotouHGeD9e5JDcxz4fZYcsg86sxfxHkxx433cdmP6HrAEk qXrc+urtRiZjqJvUeYAlKqfJU2O5oGcJus9uZ5ePCueSrJA59iEYM99IQcRr4PxDlH KR2TZMdSym3GbI/F8CtRnK2FK8tAZzywiVpIfZwenWzAG4o8YpIIKAz4APJU/J4EH3 HuNrojgkpeT74yfUIZdMspOlhkR/XG2gAPqEGY0Cs18hnO4xiJflg7epi7NmGOFXL/ hmBUkOTy7d52Nk6wEi1mzkb4YnlyeGzr0qP/D/mOd82RQdBtsyfCX6dPhhUmwM5o0i MKglDcXVpi3pg== Message-ID: <43cc2290-10b6-4db3-bfc0-169adb8201b7@kernel.org> Date: Tue, 24 Mar 2026 13:46:20 +0100 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/memory: fix PMD/PUD checks in follow_pfnmap_start() To: "Lorenzo Stoakes (Oracle)" Cc: linux-kernel@vger.kernel.org, Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Peter Xu , linux-mm@kvack.org, Alex Williamson , Max Boone , stable@vger.kernel.org References: <20260323-follow_pfnmap_fix-v1-1-5b0ec10872b3@kernel.org> 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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 3/24/26 12:04, Lorenzo Stoakes (Oracle) wrote: > On Mon, Mar 23, 2026 at 09:20:18PM +0100, David Hildenbrand (Arm) wrote: >> follow_pfnmap_start() suffers from two problems: >> >> (1) We are not re-fetching the pmd/pud after taking the PTL >> >> Therefore, we are not properly stabilizing what the lock lock actually >> protects. If there is concurrent zapping, we would indicate to the >> caller that we found an entry, however, that entry might already have >> been invalidated, or contain a different PFN after taking the lock. >> >> Properly use pmdp_get() / pudp_get() after taking the lock. >> >> (2) pmd_leaf() / pud_leaf() are not well defined on non-present entries >> >> pmd_leaf()/pud_leaf() could wrongly trigger on non-present entries. >> >> There is no real guarantee that pmd_leaf()/pud_leaf() returns something >> reasonable on non-present entries. Most architectures indeed either >> perform a present check or make it work by smart use of flags. > > It seems huge page split is the main user via pmd_invalidate() -> > pmd_mkinvalid(). > > And I guess this is the kind of thing you mean by smart use of flags, for > x86-64: Exactly. [...] > >> >> However, for example loongarch checks the _PAGE_HUGE flag in pmd_leaf(), >> and always sets the _PAGE_HUGE flag in __swp_entry_to_pmd(). Whereby >> pmd_trans_huge() explicitly checks pmd_present(), pmd_leaf() does not >> do that. > > But pmd_present() checks for _PAGE_HUGE in pmd_present(), and if set checks > whether one of _PAGE_PRESENT, _PAGE_PROTNONE, _PAGE_PRESENT_INVALID is set, > and pmd_mkinvalid() sets _PAGE_PRESENT_INVALID (clearing _PAGE_PRESENT, > _VALID, _DIRTY, _PROTNONE) so it'd return true. pmd_present() will correctly indicate "not present" for, say, a softleaf migration entry. However, pmd_leaf() will indicate "leaf" for a softleaf migration entry. So not checking pmd_present() will actually treat non-present migration entries as present leafs in this function, which is wrong in the context of this function. We're walking present entries where things like pmd_pfn(pmd) etc make sense. > > pmd_leaf() simply checks to see if _PAGE_HUGE is set which should be > retained on split so should all still have worked? > > But anyway this is still worthwhile I think. > >> >> Let's check pmd_present()/pud_present() before assuming "the is a >> present PMD leaf" when spotting pmd_leaf()/pud_leaf(), like other page >> table handling code that traverses user page tables does. >> >> Given that non-present PMD entries are likely rare in VM_IO|VM_PFNMAP, >> (1) is likely more relevant than (2). It is questionable how often (1) >> would actually trigger, but let's CC stable to be sure. >> >> This was found by code inspection. >> >> Fixes: 6da8e9634bb7 ("mm: new follow_pfnmap API") >> Cc: stable@vger.kernel.org >> Signed-off-by: David Hildenbrand (Arm) > > This looks correct to me, so: > > Reviewed-by: Lorenzo Stoakes (Oracle) Thanks! > >> --- >> Gave it a quick test in a VM with MM selftests etc, but I am not sure if >> I actually trigger the follow_pfnmap machinery. >> --- >> mm/memory.c | 18 +++++++++++++++--- >> 1 file changed, 15 insertions(+), 3 deletions(-) >> >> diff --git a/mm/memory.c b/mm/memory.c >> index 219b9bf6cae0..2921d35c50ae 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -6868,11 +6868,16 @@ int follow_pfnmap_start(struct follow_pfnmap_args *args) >> >> pudp = pud_offset(p4dp, address); >> pud = pudp_get(pudp); >> - if (pud_none(pud)) >> + if (!pud_present(pud)) >> goto out; >> if (pud_leaf(pud)) { >> lock = pud_lock(mm, pudp); >> - if (!unlikely(pud_leaf(pud))) { >> + pud = pudp_get(pudp); >> + >> + if (unlikely(!pud_present(pud))) { >> + spin_unlock(lock); >> + goto out; >> + } else if (unlikely(!pud_leaf(pud))) { > > Tiny nit, but no need for else here. Sometimes compilers complain about > this but not sure if it such pedantry is enabled in default kernel compiler > flags :) You mean if (unlikely(!pud_present(pud))) { spin_unlock(lock); goto out; } if (...) { ? That just creates an additional LOC without any benefit IMHO. And we use it all over the place :) In fact, I will beat any C compiler with the C standard that complains about that ;) -- Cheers, David