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.
Tested-by: Michal Suchánek <[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]/ > 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(). > */ > i = si_mem_available(); > - if (i < nr_pages) > + if (system_state != SYSTEM_BOOTING && i < nr_pages) > return -ENOMEM; > > /* > -- > 2.54.0 (Apple Git-157) > >
