Re: [PATCH v2] ring-buffer: Do not check si_mem_available() before deferred init completes

[Amit Machhiwal <amachhiw@linux.ibm.com>]

On 2026/10/19 12:57 PM, Michal Suchánek wrote:
> On Fri, Oct 16, 2026 at 07:12:47PM +0530, Amit Machhiwal wrote:
> > During early boot (early_trace_init() called from start_kernel()),
> > tracing allocates initial ring buffers (temp_buffer, array_buffer, and
> > snapshot_buffer) for CPU 0.  Later, as secondary CPUs come online during
> > smp_init(), each CPU's ring buffer is allocated via trace_rb_cpu_prepare().
> > 
> > Before attempting page allocation, __rb_allocate_pages() performs a
> > heuristic check using si_mem_available() to return early with -ENOMEM if
> > memory appears insufficient.
> > 

<snip>

> >  kernel/trace/ring_buffer.c | 14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> > 
> > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> > index 04bb94c29f58..aa877a272c89 100644
> > --- a/kernel/trace/ring_buffer.c
> > +++ b/kernel/trace/ring_buffer.c
> > @@ -2452,10 +2452,24 @@ static int __rb_allocate_pages(struct ring_buffer_per_cpu *cpu_buffer,
> >  	 * memory. It may not be accurate. But we don't care, we just want
> >  	 * to prevent doing any allocation when it is obvious that it is
> >  	 * not going to succeed.
> > +	 *
> > +	 * With CONFIG_DEFERRED_STRUCT_PAGE_INIT, NR_FREE_PAGES only reflects
> > +	 * the initial non-deferred pool during early boot and throughout SMP
> > +	 * bring-up (smp_init() runs before page_alloc_init_late() completes
> > +	 * deferred initialization). si_mem_available() returns a false negative
> > +	 * across this entire window while actual allocations succeed by growing
> > +	 * the zone on demand via deferred_grow_zone(). Only re-enable the check
> > +	 * from SYSTEM_FREEING_INITMEM onward, after page_alloc_init_late() has
> > +	 * finished and si_mem_available() is accurate.
> >  	 */
> >  	i = si_mem_available();
> > +#ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
> > +	if (system_state >= SYSTEM_FREEING_INITMEM && i < nr_pages)
> > +		return -ENOMEM;
> > +#else
> >  	if (i < nr_pages)
> >  		return -ENOMEM;
> > +#endif
> >  
> >  	/*
> >  	 * If a user thread allocates too much, and si_mem_available()
> > 
> > base-commit: 2f2f7eeef8b0a1a9b9ca998231419873ccae8d4e
> 
> Hello,
> 
> due to other conflicting changes this no longer applies.
> 
> Will you send a rebase?

Hi Michal,

Thanks for checking and notifying.  I'll look into it and send a v3 as needed.

Thanks,
Amit