On 2017-01-26 14:26:12 -0800, Stephane Eranian wrote: > On Thu, Jan 26, 2017 at 2:19 PM, Peter Zijlstra <[email protected]> wrote: > > On Thu, Jan 26, 2017 at 01:00:52PM -0800, Andres Freund wrote: > >> The problem is that (quoted below) without that hack the subsequent > >> mmaps just expand the previous VMAs which leads to perf loosing its > >> (address,range) -> symbol mappings for previously (in the same expanded > >> range) known symbols. > > > > I'm assuming this is the userspace tool doing that, right? > > > > Acme, is that something we can cure? Instead of throwing away eveything > > known of the previous map, merge them when they grow? > > > > So I think the problem was that we don't track munmap(), so if you see > > two overlapping mappings, we have no clue and assume an munmap() has > > happened or something. I can't really remember, its been many many years > > since I wrote all that :-( > > But perf report takes care of overlapping mmaps normally, > see util/map.c:map_groups__fixup_overlappings > > VMA merging does not change anything at the user level. > The kernel will see two mmaps and so will perf record and perf report.
But the kernel *didn't* report two independent mmaps. Looking at strace shows that the mmaps were done independently in page sized chunks: 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 (first function) 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 (second function) But then looking at the unannotated perf script --show-mmap-events: 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 these four mmaps regions are: 0x7efd3866e000(0x1000): 0x7efd3866e000 - 0x7efd3866f000 0x7efd3866d000(0x2000): 0x7efd3866d000 - 0x7efd3866f000 0x7efd3866c000(0x3000): 0x7efd3866c000 - 0x7efd3866f000 0x7efd3866b000(0x4000): 0x7efd3866b000 - 0x7efd3866f000 So there's just a single mapping being reported. > This is what you have. I'm a bit confused - isn't the trace showing exactly that the mmaps *are* reported with a merged VMA? > In the case of Java, if I recall, there is a single large mmap for the > code cache and it works fine. That explains why it's working there... Greetings, Andres Freund
