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=1794004972; b=JXG1PBX5RP0RCe5wt7Br4pwh4Gpy39HiJ7uI/WdwePoHLRshk1qCu7erRiiT4W0tfvFM 1NqHuc9JrkOB0C78+xN74Z0shHZ/agYDgUMbLy0cf+J19QdQF/BRPDbd/8Qa4uy4sSRuO MAogBlNcfdQytUOgainsj6PvJTSB0UVNlUmvtaCr2Xq+66keRYxqWTstjTUhUoOiUt294 N2hAPhWI2tnpX0BN3w7+WHRI6uwMZn8KD58AM/9myu3yrg/u5e4cZ1fM+oj9IV8AXF1rl oj/D2X80WkLvrt46IJ0VIKiXmNg5pDPtdpOkp2D+GdFI5DYE1OdeqVfJ00KGK84TIcA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794004972; 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=xrGWYrqGixJhZjc6cA3u15ineEoMPVTqbCNBS3vO/E8=; b=1mIslP96p1kLHlqhA3763SVe9k1E0Nxjp4PLV1S4ICSUa/TT9KM4+L9cIF/YXhHoyuSA oIDkh0mkqEJPT64SMM+2VZMLbnCCOKzseejPjc7mV2VFkouCPVQe7N8q6YRj5h0CnOsnK 45AYZxbf+vaYqhuIiZ0k0TL6BbeS0nEzYwwpeqHlUKa0BAXbexxACHU0QwFP5NT4t7EpB NR5bJR0SEb1t/6jBmGhKxAVct4T+dPAG8Exp4+1Oi/7vP78fN5Oo74CHCY7VRavgpOlE4 09RKMHp3w4ZbA5MHzVX2V0a60xi15Nf2MA92g5Xi9ayYLw8tzzWC3HnjbF1321mn5Lg== 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=L63UwzrP; 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 EA998606A326; Fri, 6 Nov 2026 22:42:51 +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=L63UwzrP Received: by smtp.kernel.org (Postfix) with ESMTPSA id 019D31F0089F; Fri, 6 Nov 2026 22:42:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794004971; bh=xrGWYrqGixJhZjc6cA3u15ineEoMPVTqbCNBS3vO/E8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=L63UwzrPS2ataztpBxby6ngbB/ads9+t+yjRipL316Dhw0VSCOCocujlpDrXQuR92 8cHpI5LF473Ryale/9WffhFi44DKGSYK/ZKvCx/DE/l3c1ItJmN6w0NuBhoDgnTVim R0DJ5yQCaHR1w6HEmo9gpud9qnI/d42WwKGteXUoEtngqrw8LxtvSBXHqCXAOCRQK8 DCWqGNxp8pGnD9fk5PGtMcnSu4BYj3TXiWyH31as3EUW4+fGpL7NWJmAsPyq4O1QVu UWgmOZlW8cQq63VBB2npIDRUKf9QYGivudLkoE0YWyW/Nw19B2w3UDUxxAcWiNZ6/d Zm3KgmdtNVSrQ== Message-ID: <9c86902c-b942-4641-aee0-672adcc84823@kernel.org> Date: Fri, 6 Nov 2026 23:42:17 +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 5/5] mm/gup: limit pagecache folio refcount when grabbing references To: "Jose A. Perez de Azpillaga" , 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 , Kiryl Shutsemau , 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 Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, iommu@lists.linux.dev, stable@vger.kernel.org References: <20261105-refcount_overflow_stable-v1-0-70b9c655da3e@kernel.org> <20261105-refcount_overflow_stable-v1-5-70b9c655da3e@kernel.org> <4763a039-60f8-4263-8124-beb0f9ab3366@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: <4763a039-60f8-4263-8124-beb0f9ab3366@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/6/26 11:13, Jose A. Perez de Azpillaga wrote: > On 11/5/26 2:10 PM, David Hildenbrand (Arm) wrote: >> Let's replace our existing safety check by the new >> folio_ref_count_over_limit() check in fast and slow GUP before acquiring >> additional references or pins. >> >> Keep it simple and use the check for all folio types. >> >> Drop the WARN_ON_ONCE(), because apparently these things can currently >> happen with large pagecache folios. >> >> Keep the zero-refcount rejection if the refcount is 0. >> >> 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) >> --- >> mm/gup.c | 12 ++++++++---- >> 1 file changed, 8 insertions(+), 4 deletions(-) >> >> diff --git a/mm/gup.c b/mm/gup.c >> index 2ab42fc6834e5..db9db6b2a4662 100644 >> --- a/mm/gup.c >> +++ b/mm/gup.c >> @@ -77,7 +77,7 @@ static inline struct folio *try_get_folio(struct page *page, int refs) >> >> retry: >> folio = page_folio(page); >> - if (WARN_ON_ONCE(folio_ref_count(folio) < 0)) >> + if (unlikely(folio_ref_count_over_limit(folio))) >> return NULL; >> if (unlikely(!folio_ref_try_add(folio, refs))) >> return NULL; >> @@ -140,7 +140,11 @@ static void gup_put_folio(struct folio *folio, int refs, unsigned int flags) >> int __must_check try_grab_folio(struct folio *folio, int refs, >> unsigned int flags) >> { >> - if (WARN_ON_ONCE(folio_ref_count(folio) <= 0)) >> + if ((flags & (FOLL_GET | FOLL_PIN)) && >> + unlikely(folio_ref_count_over_limit(folio))) >> + return -ENOMEM; >> + >> + if (WARN_ON_ONCE(!folio_ref_count(folio))) >> return -ENOMEM; >> >> if (unlikely(!(flags & FOLL_PCI_P2PDMA) && folio_is_pci_p2pdma(folio))) >> @@ -1482,8 +1486,8 @@ static long __get_user_pages(struct mm_struct *mm, >> struct folio *folio = page_folio(page); >> >> /* >> - * Since we already hold refcount on the >> - * large folio, this should never fail. >> + * Grabbing more references can fail once we >> + * hit folio refcount limits. >> */ >> if (try_grab_folio(folio, page_increm - 1, >> gup_flags)) { > > this can fail now, so shouldn't the error path below return -ENOMEM like > the first page does, instead of -EFAULT? Hm, indeed, that's inconsistent. The issue is that we have this -EREMOTEIO check mangled in there. But the ordinary path would already have checked that and failed with he same error. So this hunk would become: /* - * Since we already hold refcount on the - * large folio, this should never fail. + * Grabbing more references can fail once we + * hit folio refcount limits. */ - if (try_grab_folio(folio, page_increm - 1, - gup_flags)) { + ret = try_grab_folio(folio, page_increm - 1, + gup_flags); + if (ret) { /* - * Release the 1st page ref if the - * folio is problematic, fail hard. + * Release the folio reference we + * already took. */ gup_put_folio(folio, 1, gup_flags); - ret = -EFAULT; goto out; } } -- Cheers, David