RFR(xxs): 8187230: [aix] Leave OS guard page size at default for non-java threads instead of explicitly setting it

[Thomas Stüfe]

Hi all,

I am cleaning up my backlog of old issues which did not make it into the
repo before the consolidation.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8187230

Last Webrev (just rebased to the new repo structure, no changes):
http://cr.openjdk.java.net/~stuefe/webrevs/8187230-aix-leave-os-guard-page-size-at-default-for-non-java-threads/webrev.02/webrev/

For your convenience, here the original message:

<<<
The change is very subtle.

Before, we would set the OS guard page size for every thread - for java
threads disable them, for non-java threads we'd set them to 4K.

Now, we still disable them for java threads but leave them at the OS
default size for non-java threads.

The really important part is the disabling of OS guard pages for java
threads, where we have a VM guard pages in place and do not want to spend
more memory on OS guards. We do not really care for the exact size of the
OS guard pages for non-java threads, and therefore should not set it - we
should leave the size in place the OS deems sufficient. That also spares us
the complexity of handling the thread stack page size, which on AIX may be
different from os::vm_page_size().
>>>

@Chris: you did ask whether this would make sense for Linux too. I think
you are right, but as Goetz pointed out matters are more complicated as
glibc pthread_create does not substract OS guard size from the user
specified stack size, so it requires us to know the OS guard size and add
it to the specified stack size (funny, the same issue we have with VM
guards and -Xss). So, for now, I'd prefer this to keep AIX only.

I think I need a second reviewer beside Goetz.

Thanks!

Thomas



On Fri, Sep 8, 2017 at 10:48 AM, Thomas Stüfe <thomas.stuefe at gmail.com>
wrote:

> Hi Guys,
>
> On Fri, Sep 8, 2017 at 9:51 AM, Lindenmaier, Goetz <
> goetz.lindenmaier at sap.com> wrote:
>
>> Hi Chris,
>>
>> on linux the pthread implementation is a bit strange, or buggy.
>> It takes the OS guard pages out of the stack size specified.
>> We need to set it so we can predict the additional space
>> that must be allocated for the stack.
>>
>> See also the comment in os_linux.cpp, create_thread().
>>
>
> Goetz, I know we talked about this off list yesterday, but now I am not
> sure this is actually needed. Yes, to correctly calculate the stack size,
> we need to know the OS guard page size, but we do not need to set it, we
> just need to know it. So, for non-java threads (java threads get the OS
> guard set to zero), it would probably be sufficient to:
>
> - pthread_attr_init() (sets default thread attribute values to the
> attribute structure) and then
> - pthread_attr_getguardsize() to read the guard size from that structure.
>
> That way we leave the OS guard page at the size glibc deems best. I think
> that is a better option. Consider a situation where the glibc changes the
> size of the OS guard pages, for whatever reason - we probably should follow
> suit.
>
> See e.g. this security issue - admittedly only loosely related, since the
> fix for this issue seemed to be a fix to stack banging, not changing the OS
> guard size: https://access.redhat.com/security/vulnerabilities/stackguard
>
> So, in short, I think we could change this for Linux too. If you guys
> agree, I'll add this to the patch. Since I am on vacation and the depot is
> closed, it may take some time.
>
> Kind Regards, Thomas
>
>
>
>
>
>>
>> Best regards,
>>   Goetz.
>>
>> > -----Original Message-----
>> > From: ppc-aix-port-dev [mailto:ppc-aix-port-dev-bounc
>> es at openjdk.java.net]
>> > On Behalf Of Chris Plummer
>> > Sent: Thursday, September 07, 2017 11:07 PM
>> > To: Thomas Stüfe <thomas.stuefe at gmail.com>; ppc-aix-port-
>> > dev at openjdk.java.net
>> > Cc: HotSpot Open Source Developers <hotspot-dev at openjdk.java.net>
>> > Subject: Re: RFR(xxs): 8187230: [aix] Leave OS guard page size at
>> default for
>> > non-java threads instead of explicitly setting it
>> >
>> > Hi Thomas,
>> >
>> > Is there a reason this shouldn't also be done for linux?
>> >
>> > thanks,
>> >
>> > Chris
>> >
>> > On 9/7/17 3:02 AM, Thomas Stüfe wrote:
>> > > Hi all,
>> > >
>> > > may I please have a review for this small change:
>> > >
>> > > Bug:
>> > > https://bugs.openjdk.java.net/browse/JDK-8187230
>> > >
>> > > Webrev:
>> > > http://cr.openjdk.java.net/~stuefe/webrevs/8187230-aix-
>> > > leave-os-guard-page-size-at-default-for-non-java-
>> > threads/webrev.00/webrev/
>> > >
>> > > The change is very subtle.
>> > >
>> > > Before, we would set the OS guard page size for every thread - for
>> java
>> > > threads disable them, for non-java threads we'd set them to 4K.
>> > >
>> > > Now, we still disable them for java threads but leave them at the OS
>> > > default size for non-java threads.
>> > >
>> > > The really important part is the disabling of OS guard pages for java
>> > > threads, where we have a VM guard pages in place and do not want to
>> > spend
>> > > more memory on OS guards. We do not really care for the exact size of
>> the
>> > > OS guard pages for non-java threads, and therefore should not set it
>> - we
>> > > should leave the size in place the OS deems sufficient. That also
>> spares us
>> > > the complexity of handling the thread stack page size, which on AIX
>> may be
>> > > different from os::vm_page_size().
>> > >
>> > > Thank you and Kind Regards, Thomas
>> >
>> >
>>
>>
>
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