On Fri, Oct 23, 2026 at 08:24:59PM +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. > > However, on kernels built with CONFIG_DEFERRED_STRUCT_PAGE_INIT=y, > NR_FREE_PAGES only reflects the initial non-deferred pool across the entire > period from early_trace_init() through smp_init(). defer_init() leaves > only a single section per node (e.g. 16 MiB with 64 KB pages) initialized > up-front, and on kernels with large static binary footprints (debug > configurations enabling PAGE_OWNER, DEBUG_PAGEALLOC, KFENCE, or SLUB_DEBUG), > early core initialisations (SLUB caches, vmalloc, static ftrace records) > consume virtually all managed pages in this pool. > > si_mem_available() therefore returns a false negative (often 0-1 pages) > across both SYSTEM_BOOTING (for the initial CPU 0 buffers) and > SYSTEM_SCHEDULING (for secondary CPU buffers via trace_rb_cpu_prepare()), > causing __rb_allocate_pages() to prematurely abort with -ENOMEM. > > This failure is false: the buddy allocator is designed to replenish > ZONE_NORMAL on demand via deferred_grow_zone() when an actual allocation > is attempted. Checking si_mem_available() before calling alloc_pages_node() > short-circuits this on-demand growth entirely. > > deferred_grow_zone() is only permanently disabled after page_alloc_init_late() > completes (it spawns per-node kthreads and blocks until done), which happens > inside kernel_init_freeable() — before system_state is set to > SYSTEM_FREEING_INITMEM. Skip the si_mem_available() check until > SYSTEM_FREEING_INITMEM, at which point all deferred pages are initialized > and si_mem_available() is accurate. > > Use IS_ENABLED(CONFIG_DEFERRED_STRUCT_PAGE_INIT) to set i = LONG_MAX during > this window, unconditionally bypassing the threshold check. On kernels > without deferred init IS_ENABLED() compiles the branch away and the original > check is preserved unchanged.
Commit messages are not LLMs' strong suit. Otherwise. Reviewed-by: Vincent Donnefort <[email protected]> > > 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]/ > Link: https://lore.kernel.org/all/[email protected]/ > Signed-off-by: Amit Machhiwal <[email protected]> > --- > Changes since v3: > - Replace #ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT / #else / #endif with > IS_ENABLED() + i = LONG_MAX to avoid duplicating the threshold check. > (Vincent Donnefort) > - Trim the in-code comment to one sentence. (Vincent Donnefort) > > Changes since v2: > - Rebase on top of upstream ring_buffer.c changes from Vincent Donnefort; > update threshold to i < (nr_pages << subbuf_order). (Michal Suchánek) > > Changes since v1: > - Extend the guard to cover SYSTEM_SCHEDULING as well: smp_init() calls > trace_rb_cpu_prepare() -> __rb_allocate_pages() before > page_alloc_init_late() > completes, so the false negative window spans both SYSTEM_BOOTING and > SYSTEM_SCHEDULING. Use system_state >= SYSTEM_FREEING_INITMEM as the > correct > boundary. (Sashiko, Vincent Donnefort) > - Wrap in #ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT to preserve the original > check unchanged on non-deferred kernels. (Vincent Donnefort) > - Drop Tested-by: guard condition changed, requesting fresh test. > > kernel/trace/ring_buffer.c | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c > index 9c4e40847e43..09c02e838c4d 100644 > --- a/kernel/trace/ring_buffer.c > +++ b/kernel/trace/ring_buffer.c > @@ -2463,8 +2463,16 @@ 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, si_mem_available() > + * will reflect the actual available memory only after > + * SYSTEM_FREEING_INITMEM. > */ > i = si_mem_available(); > + if (IS_ENABLED(CONFIG_DEFERRED_STRUCT_PAGE_INIT) && > + system_state < SYSTEM_FREEING_INITMEM) > + i = LONG_MAX; > + > if (i < (nr_pages << subbuf_order)) > return -ENOMEM; > > > base-commit: 734bef548a96e8774d6eef5dfa2026c6edc4e80b > -- > 2.54.0 (Apple Git-157) >
