Hi Vincent,
Thanks for reviewing this version. Please find my response inline below.
On 2026/10/23 09:20 AM, Vincent Donnefort wrote:
> On Fri, Oct 23, 2026 at 09:18:10AM +0100, Vincent Donnefort wrote:
> > On Thu, Oct 22, 2026 at 11:48:17PM +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.
> > >
> > > Wrap the guard in #ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT so that on
> > > kernels
> > > without deferred init the original check is preserved unchanged.
> > >
> > > 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]>
> >
> > It is usually good practice to not use in-reply-to to link the series
> > between
> > each other.
Noted, thanks. Since the patch 1 from this series was already picked, I thought
keeping the new versions of patch 2 may make sense. But will send v4 as a
standalone thread.
> >
> > > ---
> > > 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 | 17 +++++++++++++++++
> > > 1 file changed, 17 insertions(+)
> > >
> > > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> > > index 9c4e40847e43..8e5a59a7b69b 100644
> > > --- a/kernel/trace/ring_buffer.c
> > > +++ b/kernel/trace/ring_buffer.c
> > > @@ -2463,10 +2463,27 @@ 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.
> >
> > This is quite a massive comment for something simple: When
> > CONFIG_DEFERRED_STRUCT_PAGE_INIT, si_mem_available() will reflect the actual
> > available memory only after SYSTEM_FREEING_INITMEM.
Agreed, will trim it down to that one sentence.
> >
> > > */
> > > i = si_mem_available();
> > > +#ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
> > > + if (system_state >= SYSTEM_FREEING_INITMEM &&
> > > + i < (nr_pages << subbuf_order))
> > > + return -ENOMEM;
> > > +#else
> > > if (i < (nr_pages << subbuf_order))
> > > return -ENOMEM;
> > > +#endif
> >
> > Could we avoid the double i < (nr_pages << subbuf_order) with something like
> >
> > if (IS_ENABLED(CONFIG_DEFERRED_STRUCT_PAGE_INIT) &&
> > system_state < SYSTEM_FREEING_INITMEM)
> > i = LONG_MAX;
> >
>
> or even i = totalram_pages()?
The IS_ENABLED() pattern is cleaner, will adopt it. However, totalram_pages()
is not safe as the sentinel here.
_totalram_pages is set by a single totalram_pages_add() call inside
memblock_free_all(), which accumulates the total non-deferred page count from
free_low_memory_core_early() and adds it in one shot. The deferred pages freed
later by deferred_free_pages() -> __free_pages_core(MEMINIT_EARLY) take the else
branch in __free_pages_core() which only updates zone->managed_pages directly —
totalram_pages_add() is never called for them (the kernel comment there even
says "memblock adjusts totalram_pages() manually"). As a result,
totalram_pages() returns the same small initial-section value (~16 MiB on
ppc64le) throughout the entire window, and setting i = totalram_pages() would
still trigger the i < (nr_pages << subbuf_order) check falsely.
LONG_MAX is unambiguous — it unconditionally passes the threshold check
regardless of nr_pages or subbuf_order, which is exactly the intent. Will use
that in v4.
Thanks,
Amit
