Re: [PATCH] perf record: Skip affinity for per-thread mmap buffers
From: Paul Moses
Date: Fri Nov 06 2026 - 02:33:13 EST
On Friday, November 6th, 2026 at 1:18 AM, Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> Hello,
>
> On Fri, Oct 30, 2026 at 06:59:28AM +0000, Paul wrote:
> > From: Paul Moses <p@xxxxxxx>
> >
> > Per-thread mmap buffers use CPU -1 to mean any CPU. Combining --per-thread
> > with --affinity=cpu passes this value to __set_bit() when constructing the
> > reader affinity mask, causing an out-of-bounds bitmap access and SIGSEGV.
> >
> > Program received signal SIGSEGV, Segmentation fault.
> > 0x0000555555f3488e in mmap.mmap ()
> > #0 0x0000555555f3488e in mmap.mmap ()
> > #1 0x0000555555e2f7bf in mmap_per_evsel (evlist=evlist@entry=0x7fffedcc1150, ops=ops@entry=0x7fffffffaf50, idx=idx@entry=0, mp=mp@entry=0x7fffffffaf70, cpu_idx=cpu_idx@entry=0, thread=thread@entry=0, _output=0x7fffffffaf08, _output_overwrite=0x7fffffffaf04, nr_mmaps=0x7fffffffaf0c) at evlist.c:598
> > #2 0x0000555555e2ef96 in mmap_per_thread (evlist=0x7fffedcc1150, ops=0x7fffffffaf50, mp=0x7fffffffaf70) at evlist.c:650
> > #3 perf_evlist__mmap_ops (evlist=0x7fffedcc1150, ops=0x7fffffffaf50, mp=0x7fffffffaf70) at evlist.c:748
> > #4 0x0000555555f2150d in evlist.mmap_ex ()
> > #5 0x0000555555db0b0a in record.open ()
> > #6 0x0000555555dadea7 in __cmd_record ()
> > #7 0x0000555555daca89 in cmd_record ()
> > #8 0x0000555555e27d53 in run_builtin ()
> > #9 0x0000555555e277cf in main ()
>
> Please indent terminal outpus otherwise git will eat up lines start with
> the hash (#).
>
> > rip 0x555555f3488e 0x555555f3488e <mmap.mmap+382>
> > rax 0x7fffef9e9bd0 140737213537232
> > rbx 0x7fffedd249a0 140737183369632
> > rcx 0x3ffffffffffffff 288230376151711743
> > rdx 0x8000000000000000 -9223372036854775808
> > rsi 0x0 0
> > rdi 0x7fffef9e9bd0 140737213537232
> > 0x555555f34876 <mmap__mmap+358>: movswq 0x10(%rbx),%rcx
> > 0x555555f3487b <mmap__mmap+363>: mov 0xb8(%rbx),%rax
> > 0x555555f34882 <mmap__mmap+370>: mov $0x1,%edx
> > 0x555555f34887 <mmap__mmap+375>: shl %cl,%rdx
> > 0x555555f3488a <mmap__mmap+378>: shr $0x6,%rcx
> > => 0x555555f3488e <mmap__mmap+382>: or %rdx,(%rax,%rcx,8)
> > 0x555555f34892 <mmap__mmap+386>: lea 0x17711b(%rip),%r14 # 0x5555560ab9b4 <verbose>
> > 0x555555f34899 <mmap__mmap+393>: cmpl $0x2,(%r14)
> > 0x555555f3489d <mmap__mmap+397>: jne 0x555555f348f7 <mmap__mmap+487>
> > 0x555555f3489f <mmap__mmap+399>: mov 0xb8(%rbx),%rdi
> > 0x555555f348a6 <mmap__mmap+406>: mov 0xc0(%rbx),%esi
> > 0x555555f348ac <mmap__mmap+412>: lea -0x430(%rbp),%r12
> >
> > Only construct and apply reader affinity masks for buffers with a
> > nonnegative CPU. Skip the reader-side bitmap operations as well, since
> > per-thread buffers no longer allocate an affinity mask.
>
> Maybe we can just reject those options when they are used together.
>
> Thanks,
> Namhyung
>
> >
> > Fixes: 8384a2600c7d ("perf record: Adapt affinity to machines with #CPUs > 1K")
> > Signed-off-by: Paul Moses <p@xxxxxxx>
> > ---
> > tools/perf/builtin-record.c | 2 +-
> > tools/perf/util/mmap.c | 4 ++--
> > 2 files changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> > index a57987851cf0..d7c8f490cb62 100644
> > --- a/tools/perf/builtin-record.c
> > +++ b/tools/perf/builtin-record.c
> > @@ -1565,7 +1565,7 @@ static struct perf_event_header finished_init_event = {
> >
> > static void record__adjust_affinity(struct record *rec, struct mmap *map)
> > {
> > - if (rec->opts.affinity != PERF_AFFINITY_SYS &&
> > + if (map->core.cpu.cpu >= 0 && rec->opts.affinity != PERF_AFFINITY_SYS &&
> > !bitmap_equal(thread->mask->affinity.bits, map->affinity_mask.bits,
> > thread->mask->affinity.nbits)) {
> > bitmap_zero(thread->mask->affinity.bits, thread->mask->affinity.nbits);
> > diff --git a/tools/perf/util/mmap.c b/tools/perf/util/mmap.c
> > index 358e70c4f3ed..be3dffdaede8 100644
> > --- a/tools/perf/util/mmap.c
> > +++ b/tools/perf/util/mmap.c
> > @@ -292,8 +292,8 @@ int mmap__mmap(struct mmap *map, struct mmap_params *mp, int fd, struct perf_cpu
> > return -1;
> > }
> >
> > - if (mp->affinity != PERF_AFFINITY_SYS &&
> > - perf_mmap__setup_affinity_mask(map, mp)) {
> > + if (cpu.cpu >= 0 && mp->affinity != PERF_AFFINITY_SYS &&
> > + perf_mmap__setup_affinity_mask(map, mp)) {
> > pr_debug2("failed to alloc mmap affinity mask, error %d\n",
> > errno);
> > return -1;
> > --
> > 2.56.GIT
> >
> >
>
Hi Namhyung,
Can we please get this one settled first. There have been issues
with important patches being overlooked for months.
https://lore.kernel.org/linux-perf-users/20261020171946.3593127-1-p@xxxxxxx/
Thanks
Paul