perf/jit doesn't cope well with mprotect() to jit containing pages

[Andres Freund]

Hi,

While working on optionally jit-compiling parts of postgres using llvm
(MCJIT currently, but Orc would have the same issue afaics), I'm trying
to use perf jit support to make profiling of those JITed parts easier.

Turns out the current jit support in perf doesn't work that well for
LLVM - but it doesn't primarily look like LLVM's fault. Syscall-wise
llvm does (heavily filtered):

mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7efd3866e000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7efd3866d000
mprotect(0x7efd3866e000, 4096, PROT_READ|PROT_EXEC) = 0
mprotect(0x7efd3866d000, 4096, PROT_READ|PROT_EXEC) = 0
write(2, "Function loaded: evalexpr0 at 139626038091776 0x7efd3866e000 len 69", 68) = 68

mmap(0x7efd3866f000, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7efd3866c000
mmap(0x7efd3866e000, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7efd3866b000
mprotect(0x7efd3866c000, 4096, PROT_READ|PROT_EXEC) = 0
mprotect(0x7efd3866b000, 4096, PROT_READ|PROT_EXEC) = 0
write(2, "Function loaded: evalexpr1 at 139626038083584 0x7efd3866c000 len 69", 68) = 68

...

i.e. it mmaps single pages for the each JITed function's sections. Which
makes sense, because the first function is JITed independently from the
second one.

The corresponding MMAP2 records according to perf perf script
--show-mmap-events are:
        postgres  4107 595444.867737: PERF_RECORD_MMAP2 4107/4107: [0x7efd3866e000(0x1000) @ 0x7efd3866e000 00:00 0 0]: ---p //anon
        postgres  4107 595444.867825: PERF_RECORD_MMAP2 4107/4107: [0x7efd3866d000(0x2000) @ 0x7efd3866d000 00:00 0 0]: ---p //anon
        postgres  4107 595444.884090: PERF_RECORD_MMAP2 4107/4107: [0x7efd3866c000(0x3000) @ 0x7efd3866c000 00:00 0 0]: ---p //anon
        postgres  4107 595444.884113: PERF_RECORD_MMAP2 4107/4107: [0x7efd3866b000(0x4000) @ 0x7efd3866b000 00:00 0 0]: ---p //anon
Note how the size of the mapping continually increases, so that the each
MMAP2 record covers previous sections.

If one perf inject --jit into that it looks like:
        postgres  4107 595444.867737: PERF_RECORD_MMAP2 4107/4107: [0x7efd3866e000(0x1000) @ 0x7efd3866e000 00:00 0 0]: ---p //anon
        postgres  4107 595444.867825: PERF_RECORD_MMAP2 4107/4107: [0x7efd3866d000(0x2000) @ 0x7efd3866d000 00:00 0 0]: ---p //anon
        postgres  4107 595444.868140: PERF_RECORD_MMAP2 4107/4107: [0x7efd3866e000(0x45) @ 0x40 fd:02 33434534 1]: --xs /home/andres/.debug/jit/llvm-IR-jit-20161209.XXfN0K3O/jitted-4107-1.so
        postgres  4107 595444.884090: PERF_RECORD_MMAP2 4107/4107: [0x7efd3866c000(0x3000) @ 0x7efd3866c000 00:00 0 0]: ---p //anon
        postgres  4107 595444.884113: PERF_RECORD_MMAP2 4107/4107: [0x7efd3866b000(0x4000) @ 0x7efd3866b000 00:00 0 0]: ---p //anon
        postgres  4107 595444.884232: PERF_RECORD_MMAP2 4107/4107: [0x7efd3866c000(0x45) @ 0x40 fd:02 33434599 1]: --xs /home/andres/.debug/jit/llvm-IR-jit-20161209.XXfN0K3O/jitted-4107-2.so

Note how the first injected record is also covered by the following
"//anon" event.  This leads to the the curious effect that samples for
the first function (evalexpr0) are associated with the right generated
.so, until the second function is JITed.

I hacked up perf inject to omit such MMAP2 records by adding
	if (event->mmap2.prot == 0)
		return 0;
to perf_event__jit_repipe_mmap2() and suddenly things work.

I presume the increasing MMAP2 size is triggered by the consecutive
pages being represented as a single page-range in the kernel?

If I, to work around such consecutive pages, force another page to be
mmap()ed inbetween, and avoid using MAP_ANONYMOUS, the problem also goes
away.

Am I doing something wrong, or is there a bug here?

FWIW, this is on linux 4.8.8, with perf from master
(v4.9-rc8-108-g810ac7b7558d).

BTW, it's also a bit weird that those MMAP2 records triggered by
mprotect/mmap, have prot set to 0...

Regards,

Andres