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 E2B8449E5D0; Wed, 14 Oct 2026 08:12:13 +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=1791965537; cv=none; b=AG4uz97obQ8zfssWT/l+YUMiVeTGCUADoFp37vX97ZraZjOxVAlbxRQAQHEQsriYYJVMCfgPuI8gzLWGeJWFVNWRTv4+1atQRA+Wq9WL8HnM5qNa1E4E1zjRiToYlsLR7xRspICF5h6EjNfgm1/5+QV9qnv3epjEH4wJJK0W2o8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791965537; c=relaxed/simple; bh=TlCfKWp5oH9BwqHtatiQU/mvDc0K3udy3jxuQ/8Ca3I=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=XcAJU/PXVmwJBCO//T3akjx92kHGACRgJsqng+DNfc1A6cF0/ZGicEdfiKTBt8hQL2WcNZNq4vdMoL2wbztr6I7BxJLsFjJIgcJJpNzk1IJiZAu4+XKBpTAHPDxAQpt+a4yP7gbROksG0IorTC9xUjAQduyvnwxJ8OSeywSA84o= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Tpkb7ata; 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="Tpkb7ata" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 408071F00893; Wed, 14 Oct 2026 08:12:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791965533; bh=kjyStItez/lrqfbwrXHGoAACE+/nkxndtKi9r0jfGSc=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Tpkb7ataYiB/8Njw/Xj3hVOnUVPs++6DaKbTyUJBA9VMD0aq9yUZKXe0/hlKuEpTY mnUKX5xn+csTxxy/t1qlythXN0JwfhJWmIa7GxyN1Irq36gpBnxqWEHQfkkWKON9tG LrHSm4oY69zjlKpCTaczIRAgC8jzDLN97l3wH09AyqUoz7Z/kI6FCBByn1ABJad8EH T81WqRh0QCnjpaZOxAymGPBM2pMex0p1HVd9TqYf37voreoS57S9IVZC+6zXfXD4yo WkgZWKYMGwoiNz3TghhRXYd+UY/DQ4KZKDBxHmXVN9iHLfREEAnvz2m0hj+3T+CABX fkNTcgoIA3yBw== Message-ID: <9add8400-66c1-47e9-b5af-7a4081ebf3c4@kernel.org> Date: Wed, 14 Oct 2026 10:12:06 +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: fix address wraparound when scanning large folio PTEs To: "Lorenzo Stoakes (ARM)" Cc: Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Ying Huang , Baolin Wang , Barry Song , Ryan Roberts , Chuanhua Han , Kiryl Shutsemau , Kyle Zeng , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20261013-folio_pte_scan_range-v1-1-c50dc30bf6ad@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 >> Reported-by: Kyle Zeng >> Fixes: d2136d749d76 ("mm: support multi-size THP numa balancing") >> Fixes: 508758960b8d ("mm: swap: entirely map large folios found in swapcache") >> Cc: stable@vger.kernel.org >> Signed-off-by: David Hildenbrand (Arm) >> --- >> mm/memory.c | 43 +++++++++++++++++++++++++++++-------------- >> 1 file changed, 29 insertions(+), 14 deletions(-) >> >> diff --git a/mm/memory.c b/mm/memory.c >> index 1f5d5f7d39cd2..a3823010aa981 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -4933,6 +4933,27 @@ static void check_swap_exclusive(struct folio *folio, swp_entry_t entry, >> } while (--nr_pages); >> } >> >> +/* >> + * Get the address range in which a folio might be mapped, assuming its pages >> + * are mapped consecutively and the folio page at the given idx is mapped at >> + * the given address. >> + */ > > I know it's a static function but kdoc here might be nice given the number of > params, including 2 output parameters. I had that, but decided "not worth it for an internal helper". Is anything here really that confusing to understand in mm/memory.c context? I highly doubt it. > > Also then can specify exclusive end, etc. > >> +static void folio_pte_scan_range(const struct folio *folio, >> + const struct vm_area_struct *vma, unsigned long addr, >> + unsigned long idx, unsigned long *start, unsigned long *end) > > I don't love the name - to me it implies to me that you're scanning the range. > > Maybe clamp_folio_pte_range()? I guess though clamp implies it limits an input > range. Ugly. > > Maybe get_folio_pte_range()? I can just call it "folio_pte_range()". Slightly similar (but different) to "folio_pte_batch()" > > Also I think it might be useful to have it return the number of pages contained > within the range? > > Then this below: > > + folio_pte_scan_range(folio, vma, address, idx, &folio_start, > + &folio_end); > + if (unlikely((folio_end - folio_start) >> PAGE_SHIFT != nr)) > > Could be: > > + if (unlikely(get_folio_pte_range(folio, vma, address, idx, > + &folio_start, &folio_end)) != nr) > > OTOH, that's pretty ugly... :)) Not a fan of these semantics. > >> +{ >> + /* Stay within the page table and VMA boundaries. */ >> + const unsigned long pmd_start = addr & PMD_MASK; >> + const unsigned long pmd_end = pmd_start + PMD_SIZE; > > OK so get the range of the PTE page table. > >> + const unsigned long lower = max(pmd_start, vma->vm_start); > > Then clamp to VMA start... > >> + const unsigned long upper = min(pmd_end - 1, vma->vm_end - 1) + 1; > > This is trickier :) > > I guess you're doing this in case either bound is right at wraparound > somehow? > > But then couldn't the +1 push you over the edge? > > Oh wait no, because then vma->vm_end would have to equal 0, which would > fail the other-place enforced invariant that vma->vm_start < vma->vm_end. > > So it's pmd_end that could overflow, hence the -1, +1 dance. > Yes. It's fairly common, though, to handle clamping of exclusive ends that way. > Yeah I kind of hate that, that's a lot of 'just have to know' right there. > > Actually we have pmd_addr_end() already, could we use that? > > #define pmd_addr_end(addr, end) \ > ({ unsigned long __boundary = ((addr) + PMD_SIZE) & PMD_MASK; \ > (__boundary - 1 < (end) - 1)? __boundary: (end); \ > }) As replied elsewhere, I had that, but decided to open-code as pmd_addr_end() has very weird semantics with folded page tables, where we just don't clamp at all. I'll sleep over it, but to me using pmd_addr_end() is not particularly better. > > That embeds the trick in the helper and it expresses what you're getting. > > Then it could be: > > const unsigned long pmd_start = addr & PMD_MASK; > const unsigned long lower = max(pmd_start, vma->vm_start); > const unsigned long upper = pmd_addr_end(addr, vma->vm_end); > >> + const unsigned long offset = idx << PAGE_SHIFT; >> + >> + /* Avoid any address wraparounds. */ >> + *start = addr - min(addr - lower, offset); > > This is also tricky. This is clamping to max(lower, addr - offset) right, > but being very careful about underflow? > > I don't love the addr - (addr - lower) = lower here, and it feels like it > embeds a comparison there too. It's all very implicit. > > Actually maybe just naming addr - lower gets us there: > > const unsigned long max_offset = addr - lower; > ... > *start = addr - min(max_offset, offset); > > There's check_sub_overflow() or you could use a ternary also but both feel > like they add more complexity than is useful. > >> + *end = addr + min(upper - addr, folio_size(folio) - offset); > > Same deal here. > > Maybe something like: > > const unsigned long max_remaining_bytes = upper - addr; > ... > *end = addr + min(max_remaining_bytes, folio_size(folio) - offset); > Sorry, to push back, but are we overthinking this helper here? I much rather get this fixed quickly then fine tuning some details in a helper with clear semantics that anybody can understand when looking at it for 1 minute? -- Cheers, David