From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793998478; b=IFZjmGuqaTBVyQVgZXe0LRFodistEBi0wZ5LsK78gydzd9zP6MrM0Hf4h8yAn5I0L8Sb 1PkbeDPgqI30hnt+jnhkpm5NO0ps1CP7KyZxpOmBRVUX4DNmIt3WGA4eAC45bFpgJTgZX a42NJdkTeguDrp/ZewA+dQxmwAsUwr8yx/zdgbluRXxmlf89NYfj+ororHgWkA7YTZlD1 yz+GsfosX9f8FW62TqC+yxbxCQwUEfimaGVLT3kPhE4x/qGSYjcSFMTI605dZcSgxFtOP WTDra3afOhElWYRLKIyx4XnFBiD6n6pyQ4yqiZjNldGGACPgPjcjUukmGw0fdZT+U+w== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793998478; h=Received:DKIM-Signature:Message-ID:Date:MIME-Version:User-Agent: Subject:To:Cc:References:From:Content-Language:Autocrypt:In-Reply-To: Content-Type:Content-Transfer-Encoding; bh=AsX+TMzFckS6UeZpPD2RTOIgXnxB2VhmQ+PsRFL5QKo=; b=tpdAmt6J0RyVt4jvvL1rLG35wE5MJjmcH8ifJX8BwdxNNTnHQWYGr5wK25h/agt9aBNJ +2IosJ3UfiS/EBt2ksKIXJ68FFdNWXVcasxPlEtOUSTCxy+DW54R6RCZy/iOmezihIBNy UeUPs74J0ntxuHiYDp5Exs10EBoAjzCqMoJIW/TQ78XliUzG9HAyOVBg4MuqDPzjtmk8q r3JNfjT95pI48zK9mnd4P30jNbEsVpSCe2ayYs24K2XuKmxX/6LmJYS7u6nD3iGqWqIWf Wg3IHWge7ixgtafCcChcUA1BQxm2LkL31fNLRZ7uwxX3zXd7X0RBjmiG+mY5gd/B3+g== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=IpPZXKRG; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 79C17606A326; Fri, 6 Nov 2026 20:54:38 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=IpPZXKRG Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8BC9B1F008A1; Fri, 6 Nov 2026 20:54:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793998478; bh=AsX+TMzFckS6UeZpPD2RTOIgXnxB2VhmQ+PsRFL5QKo=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=IpPZXKRGpf9khfm1NTYYTBT+gxgNZ9uloqpY/GSHjqUrcXTJzZoR+hUq/DX2K7bh9 KIJjq7S0jJSkgper9xghQon8ZNFYo5LokdvH3ByRrO2r4RiqrjLdbT0dCNUJKrJRWw zZQkusqGT8tybUC3V/CG5LSebX0f9dwUH/MmI0cZOYC3lTOmUiwp+q8Qr/Ni/YSGZQ x1ZBdGNvOlz3u4N9kdHSIDNl0do2E4dd90+t4IOqsvA9fq+b9GE+mpz2Ag8RF76v6m NkL+WdGxAeY+tzEn7JJhTScK+oN4/V8Zg1dz+VOq9yDQOZVYo/zZQ9et+604MSVHHr XHUUeOum6FIXQ== Message-ID: <323500a7-ffca-4285-929e-f667415589a3@kernel.org> Date: Fri, 6 Nov 2026 21:54:24 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/5] mm: limit pagecache folio refcount when mapping pages To: Kiryl Shutsemau Cc: Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , "Matthew Wilcox (Oracle)" , Jan Kara , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Matthew Brost , Joshua Hahn , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Peter Xu , Johannes Weiner , Song Liu , Rik van Riel , Hugh Dickins , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Peter Zijlstra , Jason Gunthorpe , Kevin Tian , "Joerg Roedel (AMD)" , Robin Murphy , Steve Sistare , John Hubbard , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, iommu@lists.linux.dev, Chris Mason , stable@vger.kernel.org References: <20261105-refcount_overflow_stable-v1-0-70b9c655da3e@kernel.org> <20261105-refcount_overflow_stable-v1-1-70b9c655da3e@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 11/6/26 13:38, Kiryl Shutsemau wrote: > On Thu, Nov 05, 2026 at 02:10:10PM +0100, David Hildenbrand (Arm) wrote: >> With large file folios, overflowing the folio mapcount and refcount >> became possible. For anon folios it is currently not really an issue: >> each large anon folio can only be mapped once into a process. >> >> That's not really big news, but it's about time that we do something >> about it because some modern technology might find ways to abuse it :) >> >> One of the enablers for that was commit ddc58f27f9ee ("mm: drop tail page >> refcounting"), however, file thp support was added later. >> >> File folios can be mapped multiple times into the same process, allowing >> for processes to hold many references to the same folio. >> >> As a short-term solution, let's introduce two folio refcount limits: >> one for ordinary consumers and one for admins. Leave some headroom >> before the refcount goes negative, which we really don't want (having >> the mapcount go negative is also not expected). >> >> Start reporting SIGBUS if we run into limits. When restoring mappings >> from migration entries, instead zap the pte/pmd entry. We allow for >> slightly exceeding the limit to keep things simpler. >> >> This is far from ideal, but better than all easy alternatives we can >> backport comfortably. For now it fixes the big problem: letting >> the refcount (+mapcount) go negative and cause trouble. >> >> This is only a temporary fix, we'll follow up with something better. >> >> While the refcount checks are not required for order 0 folios, let's >> just add them for consistency: each folio_add_file_rmap*() and >> folio_dup_file_rmap*() needs a prior check. >> >> Also, just document -EHWPOISON for copy_present_ptes() while at it. >> >> Reported-by: Chris Mason >> Fixes: 800d8c63b2e9 ("shmem: add huge pages support") >> Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS") >> Fixes: 793917d997df ("mm/readahead: Add large folio readahead") >> Cc: stable@vger.kernel.org >> Signed-off-by: David Hildenbrand (Arm) >> --- >> include/linux/mm.h | 33 +++++++++++++++++++++++++++++++++ >> mm/filemap.c | 6 ++++++ >> mm/huge_memory.c | 24 ++++++++++++++++++++++-- >> mm/memory.c | 23 ++++++++++++++++++++--- >> mm/migrate.c | 11 +++++++++++ >> mm/userfaultfd.c | 4 ++++ >> 6 files changed, 96 insertions(+), 5 deletions(-) >> >> diff --git a/include/linux/mm.h b/include/linux/mm.h >> index a2dd76cc5bbfe..79ae8fc0d6bb4 100644 >> --- a/include/linux/mm.h >> +++ b/include/linux/mm.h >> @@ -8,6 +8,7 @@ >> #include >> #include >> #include >> +#include >> #include >> #include >> #include >> @@ -2228,6 +2229,38 @@ vm_fault_t finish_fault(struct vm_fault *vmf); >> #define folio_ref_zero_or_close_to_overflow(folio) \ >> ((unsigned int) folio_ref_count(folio) + 127u <= 127u) >> >> +/* Leave separate reserves for administrators and concurrent reference gains. */ >> +#define FOLIO_REF_COUNT_LIMIT (INT_MAX - (1U << 26)) >> +#define FOLIO_REF_COUNT_ADMIN_LIMIT (INT_MAX - (1U << 25)) >> + >> +/** >> + * folio_ref_count_over_limit - Check whether to refuse more folio references. >> + * @folio: The folio. >> + * >> + * Relevant for large pagecache folios (!hugetlb). For these, we have to check >> + * before adding page table mappings or other significant refcount users whether >> + * we are close to our folio ref count limit, to avoid overflows. >> + * >> + * Checking this for other folios does no harm but is mostly unnecessary. >> + * >> + * This check is an approximation; we leave some headroom such that concurrent >> + * increments, batches and temporary references are not a problem. >> + * >> + * Return: True if the caller should refuse additional references, false >> + * otherwise. >> + */ >> +static inline bool folio_ref_count_over_limit(const struct folio *folio) >> +{ >> + const unsigned int ref_count = folio_ref_count(folio); >> + >> + if (likely(ref_count < FOLIO_REF_COUNT_LIMIT)) >> + return false; >> + if (ref_count >= FOLIO_REF_COUNT_ADMIN_LIMIT) >> + return true; >> + /* Background threads must not consume admin reserves. */ >> + return (current->flags & PF_KTHREAD) || !capable(CAP_SYS_RESOURCE); >> +} >> + >> /** >> * folio_get - Increment the reference count on a folio. >> * @folio: The folio. > > Does the slow path need to be inline at all 15 call sites, GUP-fast > among them? It's not a lot of code. Do we really care? E.g., in gup.c, the compiler is free to not inline into each of the two callsites. > > Maybe only leave the likely() check in inline and move the rest into a > function call? > > Something like: > > static inline bool folio_ref_count_over_limit(const struct folio *folio) > { > const unsigned int ref_count = folio_ref_count(folio); > > if (likely(ref_count < FOLIO_REF_COUNT_LIMIT)) > return false; > return __folio_ref_count_over_limit(ref_count); > } > > bool __folio_ref_count_over_limit(unsigned int ref_count) > { > if (ref_count >= FOLIO_REF_COUNT_ADMIN_LIMIT) > return true; > /* Background threads must not consume admin reserves */ > if (current->flags & PF_KTHREAD) > return true; > return !capable(CAP_SYS_RESOURCE); > } > > Also, should this be capable_noaudit()? I am not sure a refused > reference should leave an audit record. cap_vm_enough_memory() checks > the admin reserve with CAP_OPT_NOAUDIT. > Not an expert, but capable_noaudit() sounds good. Not sure if that fully resolves sashiko's complaint in the last commit. Thanks! -- Cheers, David