Re: [PATCH v3.1] sched/isolation: Defer freeing of cpumask memblock memory to initcall

[Frederic Weisbecker <frederic@kernel.org>]

Le Thu, Jun 04, 2026 at 02:24:40PM -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:
>    <TASK>
>    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
>    </TASK>
> 
> 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 <longman@redhat.com>

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>

-- 
Frederic Weisbecker
SUSE Labs