Date: Tue, 28 Mar 2017 00:32:37 +0100 From: Steven Hartland <killing@multiplay.co.uk> To: Konstantin Belousov <kostikbel@gmail.com> Cc: "K. Macy" <kmacy@freebsd.org>, "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org> Subject: Re: Help needed to identify golang fork / memory corruption issue on FreeBSD Message-ID: <17f29342-f3c0-5940-d012-1a698e59a384@multiplay.co.uk> In-Reply-To: <20170327164905.GN43712@kib.kiev.ua> References: <20161206143532.GR54029@kib.kiev.ua> <e160381c-9935-6edf-04a9-1ff78e95d818@multiplay.co.uk> <CAHM0Q_Mg662u9D0KJ9knEWWqi9Ydy38qKDnjLt6XaS0ks%2B9-iw@mail.gmail.com> <18b40a69-4460-faf2-c0ce-7491eca92782@multiplay.co.uk> <20170317082333.GP16105@kib.kiev.ua> <180a601b-5481-bb41-f7fc-67976aabe451@multiplay.co.uk> <20170317124437.GR16105@kib.kiev.ua> <5ba92447-945e-6fea-ad4f-f58ac2a0012e@multiplay.co.uk> <20170327161833.GL43712@kib.kiev.ua> <3ec35a46-ae70-35cd-29f8-82e7cebb0eb6@multiplay.co.uk> <20170327164905.GN43712@kib.kiev.ua>
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On 27/03/2017 17:49, Konstantin Belousov wrote: > On Mon, Mar 27, 2017 at 05:33:49PM +0100, Steven Hartland wrote: >> On 27/03/2017 17:18, Konstantin Belousov wrote: >>> On Mon, Mar 27, 2017 at 12:47:11PM +0100, Steven Hartland wrote: >>>> OK now the similar but unrelated issue with signal stacks is solved I've >>>> moved back to the initial issue. >>>> >>>> I've made some progress with a reproduction case as detailed here: >>>> https://github.com/golang/go/issues/15658#issuecomment-288747812 >>>> >>>> In short it seems that having a running child, while the parent runs GC, >>>> is some how responsible for memory corruption in the parent. >>>> >>>> The reason I believe this is if I run the same GC in the parent after >>>> the child exits instead of while its running, I've been unable to >>>> reproduce the issue. >>>> >>>> As the memory segments are COW then the issue might be in VM subsystem. >>> Well, it might be, but it is a strange corruption mode to believe. >> Indeed, but would you agree the evidence seems to indicate that this may >> be the case, as otherwise I would have expected that running the GC >> after the child process has exited would have zero impact on the issue. >>>> In order to confirm / deny this I was wondering if there was a way to >>>> force a full copy of all segments for the child instead of using the COW >>>> optimisation. >>> No, there is no. By design, copying only occurs on faults, when VM >>> detects that the map entry needs copying. Doing the actual copy at fork >>> time would require writing a lot of new code. >> I noticed in vm_map_copy_entry the following: >> /* >> * We don't want to make writeable wired pages >> copy-on-write. >> * Immediately copy these pages into the new map by >> simulating >> * page faults. The new pages are pageable. >> */ >> vm_fault_copy_entry(dst_map, src_map, dst_entry, src_entry, >> fork_charge); >> >> I wondered if I could use vm_fault_copy_entry to force the copy on fork? > No, the vm_fault_copy_entry() only works with wired entries, e.g. it cannot > page in not yet touched page, and the result is also wired. > >>> Does go have FreeBSD/i386 port ? If yes, is the issue reproducable there ? >> Yes it does, I don't currently have i386 machine to test with, I'm >> assuming testing i386 on amd64 kernel, would likely not have any effect. > Only if the bug is in kernel and not in the go runtime. I am still not > convinced that the kernel is the culprit. > >>> Another blind experiment to try is to comment out call to >>> vm_object_collapse() in sys/vm/vm_map.c:vm_map_copy_entry() and see if >>> it changes anything. >> I'll do that shortly. >>> What could be quite interesting is to look at the parent and possibly >>> child address map after the error occured, using procstat -v. At >>> least for parent, this should be relatively easy to set up, just make >>> go runtime spin or pause on panic, instead of exiting, and then use >>> procstat. >> Here's both parent and child after a failure in the parent, which I obtained by putting the child in a nanosleep loop and only after successful GC call I send SIGTERM the child and reap it. procstat -v 53832 61121 PID START END PRT RES PRES REF SHD FLAG TP PATH 53832 0x400000 0x70e000 r-x 308 601 5 1 CN-- vn /root/golang/src/test5/test5 53832 0x70e000 0x951000 r-- 261 601 5 1 CN-- vn /root/golang/src/test5/test5 53832 0x951000 0x988000 rw- 31 0 1 0 C--- vn /root/golang/src/test5/test5 53832 0x988000 0x9ab000 rw- 18 0 1 0 C--- df 53832 0x800951000 0x800b51000 rw- 41 0 1 0 C--- df 53832 0x800b51000 0x800c21000 rw- 26 0 1 0 C--- df 53832 0x800c21000 0x800c71000 rw- 18 0 1 0 C--- df 53832 0x800c71000 0x800cb1000 rw- 1 0 1 0 C--- df 53832 0x800cb1000 0x800cf1000 rw- 2 0 1 0 C--- df 53832 0x800cf1000 0x800d71000 rw- 3 0 1 0 C--- df 53832 0x800d71000 0x800db1000 rw- 1 0 1 0 C--- df 53832 0x800db1000 0x800e71000 rw- 3 0 1 0 C--- df 53832 0x800e71000 0x800eb1000 rw- 1 1 1 0 ---- df 53832 0xc000000000 0xc000001000 rw- 1 1 2 0 CN-- df 53832 0xc41fff0000 0xc41fff8000 rw- 3 3 2 0 CN-- df 53832 0xc41fff8000 0xc420200000 rw- 251 0 1 0 C--- df 53832 0x7ffffffdf000 0x7ffffffff000 rwx 2 0 1 0 C--D df 53832 0x7ffffffff000 0x800000000000 r-x 1 1 28 0 ---- ph 61121 0x400000 0x70e000 r-x 308 601 5 1 CN-- vn /root/golang/src/test5/test5 61121 0x70e000 0x951000 r-- 261 601 5 1 CN-- vn /root/golang/src/test5/test5 61121 0x951000 0x988000 rw- 31 0 2 1 CN-- vn /root/golang/src/test5/test5 61121 0x988000 0x9ab000 rw- 18 18 2 1 CN-- df 61121 0x800951000 0x800b51000 rw- 41 41 2 1 CN-- df 61121 0x800b51000 0x800c21000 rw- 26 26 2 1 CN-- df 61121 0x800c21000 0x800c71000 rw- 18 18 2 1 CN-- df 61121 0x800c71000 0x800cb1000 rw- 1 1 2 1 CN-- df 61121 0x800cb1000 0x800cf1000 rw- 2 2 2 1 CN-- df 61121 0x800cf1000 0x800d71000 rw- 3 3 2 1 CN-- df 61121 0x800d71000 0x800db1000 rw- 1 1 2 1 CN-- df 61121 0x800db1000 0x800e71000 rw- 3 3 2 1 CN-- df 61121 0xc000000000 0xc000001000 rw- 1 1 2 0 CN-- df 61121 0xc41fff0000 0xc41fff8000 rw- 3 3 2 0 CN-- df 61121 0xc41fff8000 0xc420200000 rw- 251 0 1 0 C--- df 61121 0x7ffffffdf000 0x7ffffffff000 rwx 2 2 2 1 CN-D df 61121 0x7ffffffff000 0x800000000000 r-x 1 1 28 0 ---- ph Should the parent have lost the COW flag to the region starting at 0x800e71000? Regards Steve Regards Steve
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