Hi Vincent,
Thanks for reviewing this patch.
On 2026/10/12 05:38 PM, Vincent Donnefort wrote:
> On Fri, Oct 09, 2026 at 10:05:57PM +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.
> >
> > Before attempting page allocation, __rb_allocate_pages() performs a
> > heuristic check using si_mem_available() to return early with -ENOMEM if
> > memory appears insufficient.
> >
> > However, on kernels built with CONFIG_DEFERRED_STRUCT_PAGE_INIT=y,
> > defer_init() leaves only a single section per node (e.g. 16 MiB with 64
> > KB pages) initialized up-front. On kernels with large static binary
> > footprints (such as debug configurations enabling PAGE_OWNER,
> > DEBUG_PAGEALLOC, KFENCE, or SLUB_DEBUG), the static kernel image and
> > early core initialisations (SLUB caches, vmalloc, static ftrace records)
> > consume virtually all managed pages in this initial pool.
> >
> > At T=0.000000, watermarks have not yet been established
> > (totalreserve_pages = 0), so si_mem_available() returns the raw free
> > page count (often 0-1 pages). When the snapshot buffer or global trace
> > buffer attempts to allocate 2 sub-pages, si_mem_available() returns <
> > nr_pages and prematurely aborts with -ENOMEM.
> >
> > This failure is false: if the page allocation were actually attempted
> > via alloc_pages_node(), the page allocator would trigger
> > deferred_grow_zone() on demand to initialise additional deferred memory
> > sections. Checking si_mem_available() before attempting the allocation
> > short-circuits this on-demand growth.
> >
> > Skip the si_mem_available() check when system_state == SYSTEM_BOOTING.
> > Once the system transitions past early boot and page_alloc_init_late()
> > initialises all deferred memory, si_mem_available() accurately reflects
> > system-wide free memory and the check operates as intended for runtime
> > allocations.
> >
> > Fixes: 2a872fa4e9c8 ("ring-buffer: Check if memory is available before
> > allocation")
> > Cc: [email protected]
> > Reported-by: Michal Suchánek <[email protected]>
> > Closes: https://lore.kernel.org/all/[email protected]/
> > Signed-off-by: Amit Machhiwal <[email protected]>
> > ---
> > kernel/trace/ring_buffer.c | 8 +++++++-
> > 1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> > index 04bb94c29f58..a9f82e2f8fad 100644
> > --- a/kernel/trace/ring_buffer.c
> > +++ b/kernel/trace/ring_buffer.c
> > @@ -2452,9 +2452,15 @@ 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.
> > + *
> > + * Skip this check during early boot: with
> > CONFIG_DEFERRED_STRUCT_PAGE_INIT,
> > + * NR_FREE_PAGES only reflects the initial non-deferred pool at this
> > + * stage. si_mem_available() returns a false negative while actual
> > + * allocations succeed by growing the zone on demand via
> > + * deferred_grow_zone().
>
> As Sashiko pointed-out, if si_mem_available() is only ready after
> page_alloc_init_late() when CONFIG_DEFERRED_STRUCT_PAGE_INIT, then checking
> for
> system_state >= SYSTEM_FREEING_INITMEM might be more correct.
>
> Also I wonder if we don't want to guard with CONFIG_DEFERRED_STRUCT_PAGE_INIT,
> to keep the verification when CONFIG_DEFERRED_STRUCT_PAGE_INIT is unset.
Both suggestions adopted in v2 — using system_state >= SYSTEM_FREEING_INITMEM
and wrapping in #ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT.
Thanks,
Amit
