From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.subspace.kernel.org (Postfix) with ESMTP id A1CE3606A331 for ; Fri, 6 Nov 2026 23:22:40 +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=TH7OyQdi Received: by kanga.kvack.org (Postfix) id C4E536B05B0; Fri, 6 Nov 2026 18:22:39 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id BFFDD6B05B1; Fri, 6 Nov 2026 18:22:39 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id AA3316B05B2; Fri, 6 Nov 2026 18:22:39 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id 7E5FA6B05B0 for ; Fri, 6 Nov 2026 18:22:39 -0500 (EST) Received: from smtpin17.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay01.hostedemail.com (Postfix) with ESMTP id 195CE1C2DDC for ; Fri, 6 Nov 2026 23:22:39 +0000 (UTC) X-FDA: 85406270838.17.0F26AD7 Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf26.hostedemail.com (Postfix) with ESMTP id 5309E140003 for ; Fri, 6 Nov 2026 23:22:37 +0000 (UTC) Authentication-Results: imf26.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=TH7OyQdi; spf=pass (imf26.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1794007357; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=zOrS2oEsMLKAlSzqsplSP4kVfn432L2W3OnnE5qefj8=; b=Lq2rGi+Rqz3t0NsKbNkspT4cbEebCys5BmOGCSRweTVCKwWXYDyjNg00xjffixUnJP6EP4 QlMAhwZEM/CHzbljm1sxWNYk7KprVu9Eg8ec5T/82R1Aftt1A9SApTq0lwK5JBASOvx72y Ux4rDIBRccqdyhyi0aWC4dz2bwRqVBg= ARC-Authentication-Results: i=1; imf26.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=TH7OyQdi; spf=pass (imf26.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1794007357; b=wdG0aOkvRvqjPKNAgRv8X0WP+OkPFo+KlxDm0l+2XligafTO2q6VKt1xCG1PgOFeX36pSu L6hyK/zHns6uNLCcC37ORroRQb/JkrZd7yqsKx/uKJhWPPvF2WSddJljuaCyaXsZcL7TW+ oAU3aF8ac5JXxpdg3DitRCvKuggoLM4= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id 9F7E4601F8; Fri, 6 Nov 2026 23:22:36 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id A9CC71F008A1; Fri, 6 Nov 2026 23:22:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794007356; bh=zOrS2oEsMLKAlSzqsplSP4kVfn432L2W3OnnE5qefj8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=TH7OyQdiZ6kX4QUC4x1DKV7dNA3ms0ENMHCujHn6I/lEujseI/4nJj6Y04M8XaSZb PS2TwrZ1rtWzWJNakc76P4u33V0dUG5QLK2jvG4FQ0NZpa9uNHxYNkBI0O5M1I6Hkf kWNs/3+a31Oue8HLxwNem4ZvEUcXsiG9YO8gUSOiYPd4WCsBr6YJ+dW1UmTj9KWqHN jfREmzPSQqWtQK+GS0hEkTKOVzJLAtT/K3Zlyq78hb694Vl0md78Zfxewf25g/3Q0g p7NZ1wggioFiSZBMYZKW61Q5TZ3hgpAS1M6Ctsj/RxI93UNOl8JjqZQJeO3RfEhjzZ OCrHnmzQANBDA== Message-ID: Date: Fri, 6 Nov 2026 23:50:55 +0100 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 5/5] mm/gup: limit pagecache folio refcount when grabbing references To: sashiko-reviews@lists.linux.dev, 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 References: <20261105-refcount_overflow_stable-v1-0-70b9c655da3e@kernel.org> <20261105-refcount_overflow_stable-v1-5-70b9c655da3e@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 X-Stat-Signature: h5fq3cggnd1ex8whh741d7fnr317npby X-Rspamd-Server: rspam01 X-Rspam-User: X-Spam-Status: No, score=-12.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=4.0.1 X-Rspamd-Queue-Id: 5309E140003 X-HE-Tag: 1794007357-569937 X-HE-Meta: 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 j/xE1+Sp Ymr5TQIOF1p4I+3Aarl/1vh0V23eO115w+JK2iEqhESdGx8x2KTBd99Zc8jNNiKWeYsymvGZg2SRnOH0hihjjnxC1jlwEPodAJHqgOJGgg13Y384uWM+mxotRuWPudjKwpQA0OAqguVfMLB0y93Wn5mL8Ox4hXq6hGfLyeY8tu9066Nf1t92yw5/eYQlpF1JDEupWW+zu/qKzdJmyYUgWQbuTlI/LSL35/scavwJAyZ09EqTVeiu2Mr2Xfj+U3ABHAo6zk7wsy30LsqFtc7wsCF0yAB7U//HBg8ERqQB5PNP3sDcHlhMIKPouNXi+lDeTo1Ww+5KOaAMWEEzcJ5hRdMexckmeWjs0XaoK1lytI39LvsE= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On 11/6/26 14:11, sashiko-bot@kernel.org wrote: > Thank you for your contribution! Sashiko AI review found 1 potential > issue to consider: > - [High] mm/gup: try_get_folio() calls capable() with IRQs disabled in > GUP-fast > -- > > commit 9290efad216cdf3b5ee1cdfafab0a5d9928a1976 > Author: David Hildenbrand (Arm) > > mm/gup: limit pagecache folio refcount when grabbing references > > This commit replaces the negative refcount safety checks in try_get_folio() > and try_grab_folio() with folio_ref_count_over_limit() to enforce folio > refcount limits in fast and slow GUP before acquiring references or pins. > >> 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; > > [Severity: High] > Is it safe for try_get_folio() to call folio_ref_count_over_limit() in the > fast GUP path? > > In mm/gup.c, gup_fast() disables hardware interrupts across the page table > walk: > > mm/gup.c:gup_fast() { > ... > local_irq_save(flags); > nr_pinned = gup_fast_pgd_range(start, end, gup_flags, pages); > local_irq_restore(flags); > ... > } > > which calls into try_grab_folio_fast() -> try_get_folio() with IRQs > disabled: > > mm/gup.c:try_grab_folio_fast() { > ... > /* Raise warn if it is not called in fast GUP */ > VM_WARN_ON_ONCE(!irqs_disabled()); > ... > } > > Furthermore, get_user_page_fast_only() -> gup_fast() -> try_get_folio() is > invoked from hardirq and NMI contexts, such as perf_prepare_sample() -> > perf_virt_to_phys() in kernel/events/core.c. > > When folio_ref_count(folio) is between FOLIO_REF_COUNT_LIMIT and > FOLIO_REF_COUNT_ADMIN_LIMIT and current is not a kernel thread, > folio_ref_count_over_limit() in include/linux/mm.h calls > capable(CAP_SYS_RESOURCE): > > include/linux/mm.h:folio_ref_count_over_limit() { > 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); > } > > Calling capable(CAP_SYS_RESOURCE) -> ns_capable_common() -> > security_capable() in these contexts appears to cause several issues: > > 1. LSM capability hooks (such as selinux_capable() and apparmor_capable()) > perform audit logging via common_lsm_audit(), which allocates memory and > acquires spinlock_t, rwlock_t, and SLUB local_lock_t locks. On > CONFIG_PREEMPT_RT, these are sleeping rt_mutex locks, so acquiring them > with hardware interrupts disabled in gup_fast() will sleep in an invalid > atomic context. > > 2. When get_user_page_fast_only() is called from hardirq or NMI context, > acquiring non-IRQ/NMI-safe locks in the LSM, audit, or SLUB paths can > self-deadlock if the interrupted context on the same CPU already holds > one of those locks. > > 3. In security/apparmor/capability.c, audit_caps() accesses the per-CPU > audit_cache using get_cpu_var(), which only disables preemption: > > security/apparmor/capability.c:audit_caps() { > ... > ent = &get_cpu_var(audit_cache); > if (ad->subj_cred == ent->ad_subj_cred && > ktime_get_ns() <= ent->ktime_ns_expiration[cap]) { > ... > } else { > put_cred(ent->ad_subj_cred); > ent->ad_subj_cred = get_cred(ad->subj_cred); > ent->ktime_ns_expiration[cap] = ...; > } > put_cpu_var(audit_cache); > ... > } > > If an IRQ or NMI invokes get_user_page_fast_only() while the interrupted > context is inside audit_caps() on the same CPU, both contexts can race on > ent->ad_subj_cred and cause a double put_cred(). > > 4. In kernel/capability.c, ns_capable_common() updates current->flags > non-atomically: > > kernel/capability.c:ns_capable_common() { > ... > capable = security_capable(current_cred(), ns, cap, opts); > if (capable == 0) { > current->flags |= PF_SUPERPRIV; > return true; > } > ... > } > > When called from hardirq or NMI context, this mutates the interrupted > task's flags and can race with process-context updates to current->flags. > > Should try_get_folio() avoid calling capable() in fast GUP (for example, by > falling back to slow GUP once the folio refcount reaches > FOLIO_REF_COUNT_LIMIT)? What a minefiled :) Best to throw in something like if (!in_task() || irqs_disabled()) true; In the (new) slow helper to just reject admin reserves in such context. -- Cheers, David