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Wed, 01 Jul 2026 15:03:15 -0400 X-MC-Unique: H1hypGlLPHGj6D5PMJbcDw-1 X-Mimecast-MFC-AGG-ID: H1hypGlLPHGj6D5PMJbcDw_1782932593 Received: from mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.93]) (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-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 56DD21933B41; Wed, 1 Jul 2026 19:03:12 +0000 (UTC) Received: from [10.2.16.170] (unknown [10.2.16.170]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 31078180059C; Wed, 1 Jul 2026 19:03:08 +0000 (UTC) Message-ID: <5428c950-853a-4a47-9079-18a87d80e073@redhat.com> Date: Wed, 1 Jul 2026 15:03:07 -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 v3.1] sched/isolation: Defer freeing of cpumask memblock memory to initcall To: Phil Auld Cc: Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , K Prateek Nayak , Frederic Weisbecker , linux-kernel@vger.kernel.org References: <20260604182440.430811-1-longman@redhat.com> <20260701141357.GA156809@pauld.westford.csb> <20260701142559.GB156809@pauld.westford.csb> Content-Language: en-US From: Waiman Long In-Reply-To: <20260701142559.GB156809@pauld.westford.csb> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 On 7/1/26 10:25 AM, Phil Auld wrote: > On Wed, Jul 01, 2026 at 10:13:57AM -0400 Phil Auld wrote: >> Hi Waiman, >> >> On Thu, Jun 04, 2026 at 02:24:40PM -0400 Waiman Long wrote: >>> 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 saving the housekeeping cpumask memblock memory to >>> be freed into a free list in housekeeping_init() and add a new >>> housekeeping_late_init() helper to defer the actual freeing of memblock >>> memory to when initcall's are being processed. The non-atomic version >>> of the llist APIs are used as there is no contention. >>> >>> 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 >>> --- >>> kernel/sched/isolation.c | 16 +++++++++++++++- >>> 1 file changed, 15 insertions(+), 1 deletion(-) >>> >>> [v3.1] Add __initdata to memblock_freelist >>> >>> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c >>> index ef152d401fe2..156025ef81b7 100644 >>> --- a/kernel/sched/isolation.c >>> +++ b/kernel/sched/isolation.c >>> @@ -8,6 +8,7 @@ >>> * >>> */ >>> #include >>> +#include >>> #include >>> #include "sched.h" >>> >>> @@ -27,6 +28,7 @@ struct housekeeping { >>> }; >>> >>> static struct housekeeping housekeeping; >>> +static __initdata LLIST_HEAD(memblock_freelist); >>> >>> bool housekeeping_enabled(enum hk_type type) >>> { >>> @@ -189,10 +191,22 @@ void __init housekeeping_init(void) >>> WARN_ON_ONCE(cpumask_empty(omask)); >>> cpumask_copy(nmask, omask); >>> RCU_INIT_POINTER(housekeeping.cpumasks[type], nmask); >>> - memblock_free(omask, cpumask_size()); >>> + __llist_add((struct llist_node *)omask, &memblock_freelist); >> This cast is somewhat concerning. I think I see why it's needed. Wrapping >> it in a proper struct would require more allocating and freeing and >> make the problem worse. It should work though. >> >> > Fwiw, opencode/sonnet suggested a comment like this: > > /* > * We can't allocate wrapper structs from memblock as they'd need > * deferred freeing too. Instead, reuse the cpumask memory itself > * as llist nodes. This is safe because: > * - cpumask_size() >= sizeof(struct llist_node) I know that as the smallest allocation size is sizeof(long) which is the size of a llist_node. I should have mentioned that either in the commit log or as a comment. Cheers, Longman > * - Memory is properly aligned (SMP_CACHE_BYTES) > * - The cpumask is never accessed after being added to the list > */ > > ... which may be overkill :) > > > > Cheers, > Phil > > >> Reviewed-by: Phil Auld >> >> >> >> >> Cheers, >> Phil >> >> >>> } >>> } >>> >>> +static int __init housekeeping_late_init(void) >>> +{ >>> + struct llist_node *llnode, *pos, *t; >>> + >>> + /* Free allocated memblock memory, if any */ >>> + llnode = __llist_del_all(&memblock_freelist); >>> + llist_for_each_safe(pos, t, llnode) >>> + memblock_free(pos, cpumask_size()); >>> + return 0; >>> +} >>> +pure_initcall(housekeeping_late_init); >>> + >>> static void __init housekeeping_setup_type(enum hk_type type, >>> cpumask_var_t housekeeping_staging) >>> { >>> -- >>> 2.54.0 >>> >>> >> -- >> >>