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Thu, 04 Jun 2026 13:50:50 -0400 X-MC-Unique: cFaH-fSzOj2HdBKa4Y0rgw-1 X-Mimecast-MFC-AGG-ID: cFaH-fSzOj2HdBKa4Y0rgw_1780595448 Received: from mx-prod-int-10.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-10.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.95]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id DA0E61944D26; Thu, 4 Jun 2026 17:50:47 +0000 (UTC) Received: from [10.22.88.175] (unknown [10.22.88.175]) by mx-prod-int-10.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id B0C4C414; Thu, 4 Jun 2026 17:50:44 +0000 (UTC) Message-ID: <966aa306-c2c9-4078-b5b3-5ce4aeba0bf8@redhat.com> Date: Thu, 4 Jun 2026 13:50:43 -0400 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 v2] sched/isolation: Postpone re-allocation of housekeeping cpumask until initcall To: Frederic Weisbecker Cc: Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , K Prateek Nayak , linux-kernel@vger.kernel.org References: <20260602143951.266768-1-longman@redhat.com> Content-Language: en-US From: Waiman Long In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.6 on 10.30.177.95 On 6/4/26 11:24 AM, Frederic Weisbecker wrote: > Le Tue, Jun 02, 2026 at 10:39:51AM -0400, Waiman Long a écrit : >> When testing a linux-next kernel with commit 59bd1d914bb5 ("memblock: >> warn when freeing reserved memory before memory map is initialized"), >> the following warning was hit when there was a "nohz_full" kernel boot >> parameter. >> >> Cannot free reserved memory because of deferred initialization of the memory map >> WARNING: mm/memblock.c:904 at __free_reserved_area+0xde/0xf0, CPU#0: swapper/0/0 >> : >> Call Trace: >> >> memblock_phys_free+0xcb/0x100 >> housekeeping_init+0x14c/0x170 >> start_kernel+0x207/0x450 >> x86_64_start_reservations+0x24/0x30 >> x86_64_start_kernel+0xda/0xe0 >> common_startup_64+0x13e/0x141 >> >> >> IOW, we shouldn't free memblock allocated memory so early >> in the boot process when memory map isn't fully initialized in >> deferred_init_memmap(). Fix it by adding a new housekeeping_late_init() >> helper to defer the re-allocation of the housekeeping cpumasks to >> when initcall's are being processed. Also change rcu_reference() by >> rcu_reference_check() to prevent incorrect RCU lockdep splat as RCU >> will be active in this later boot stage. >> >> This commit also depends on the presence of commit 7c2eee9c1367 >> ("memblock: don't touch memblock arrays when memblock_free() is called >> late") to prevent a KASAN UAF bug report [1]. >> >> [1] https://lore.kernel.org/lkml/20260505051821.1107133-1-longman@redhat.com/ >> >> Fixes: 27c3a5967f05 ("sched/isolation: Convert housekeeping cpumasks to rcu pointers") >> Signed-off-by: Waiman Long > Thanks for fixing it. Just some points below: > >> --- >> kernel/sched/isolation.c | 20 +++++++++++++++----- >> 1 file changed, 15 insertions(+), 5 deletions(-) >> >> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c >> index ef152d401fe2..a947d75b43f1 100644 >> --- a/kernel/sched/isolation.c >> +++ b/kernel/sched/isolation.c >> @@ -164,8 +164,6 @@ int housekeeping_update(struct cpumask *isol_mask) >> >> void __init housekeeping_init(void) >> { >> - enum hk_type type; >> - >> if (!housekeeping.flags) >> return; >> >> @@ -173,17 +171,27 @@ void __init housekeeping_init(void) >> >> if (housekeeping.flags & HK_FLAG_KERNEL_NOISE) >> sched_tick_offload_init(); >> +} >> + >> +static int __init housekeeping_late_init(void) >> +{ >> + enum hk_type type; >> + >> + if (!housekeeping.flags) >> + return 0; >> + >> /* >> * Realloc with a proper allocator so that any cpumask update >> - * can indifferently free the old version with kfree(). >> + * can indifferently free the old version with kfree(). This >> + * should be done after the completion of deferred_init_memmap(). >> */ >> for_each_set_bit(type, &housekeeping.flags, HK_TYPE_MAX) { >> struct cpumask *omask, *nmask = kmalloc(cpumask_size(), GFP_KERNEL); >> >> if (WARN_ON_ONCE(!nmask)) >> - return; >> + return 0; >> >> - omask = rcu_dereference(housekeeping.cpumasks[type]); >> + omask = rcu_dereference_check(housekeeping.cpumasks[type], 1); >> >> /* We need at least one CPU to handle housekeeping work */ >> WARN_ON_ONCE(cpumask_empty(omask)); >> @@ -191,7 +199,9 @@ void __init housekeeping_init(void) >> RCU_INIT_POINTER(housekeeping.cpumasks[type], nmask); >> memblock_free(omask, cpumask_size()); >> } >> + return 0; >> } >> +pure_initcall(housekeeping_late_init); > Now that it has become an initcall, what prevents other CPUs from accessing > the housekeeping cpumasks concurrently and then dereference the free'ed memory? > > And even as a pre-smp initcall, nothing prevents interrupts or preempting tasks > from accessing them either. > > From which point on can memblock_free() be called? I see very early calls such > as print_unknown_bootoptions()... You are right. I should have just deferred the memblock_free() call. Will send a v3 to fix that. Thanks, Longman