[PATCH 0/2] tracing: Fix early boot trace buffer allocation and tracefs init crash

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

This series addresses an early boot crash and trace buffer allocation
failure reported on ppc64le KVM guests [1] (and reproducible under
CONFIG_DEFERRED_STRUCT_PAGE_INIT=y with large static kernel footprints).

Patch 1 fixes the crash (NULL pointer dereference in __find_event_file()):
When trace buffer allocation fails during early boot, tracing is left
disabled.  However, tracer_init_tracefs_work_func() runs asynchronously via
fs_initcall and traverses uninitialized event lists without checking
tracing_disabled.  Checking tracing_disabled early averts the crash.

Patch 2 fixes the root cause of the early allocation failure:
During early_trace_init(), __rb_allocate_pages() performs a heuristic check
using si_mem_available().  Under CONFIG_DEFERRED_STRUCT_PAGE_INIT=y,
NR_FREE_PAGES only reflects the initial non-deferred pool (e.g. 1 section
per node) because the remaining memory has not yet been initialized.  On
kernels with large static binary footprints (debug configs, early
SLUB/vmalloc/ftrace records), this pool is quickly consumed, causing
si_mem_available() to return 0 and trigger a false -ENOMEM before the buddy
allocator has the opportunity to grow the zone on demand via
deferred_grow_zone().  Skipping the check during SYSTEM_BOOTING allows the
allocation to succeed.

Both fixes have been verified by Michal Suchánek.

[1] https://lore.kernel.org/all/arYskzbiaNzBR9MD@kunlun.suse.cz/

Amit Machhiwal (2):
  tracing: Do not initialize tracefs work if tracing is disabled
  ring-buffer: Do not check si_mem_available() during SYSTEM_BOOTING

 kernel/trace/ring_buffer.c | 8 +++++++-
 kernel/trace/trace.c       | 6 ++++++
 2 files changed, 13 insertions(+), 1 deletion(-)


base-commit: af32da41b0327b9c6a37856ba82b6760d6c8d10e
-- 
2.54.0 (Apple Git-157)