Large page use crashes the JVM on some Linux systems

[Claes Redestad]

[ /bcc amber-dev, /cc hotspot-dev ]

Hi,

unconditionally mapping and unmapping a large page on startup seems
sub-optimal to me - could this be checked directly after
-XX:+UseLargePages flag has been parsed?

I'd also note that explicitly configured large pages are typically a 
limited
resource: does this test distinguish between a failure due the system not
supporting the feature and a failure due not having any free pages left?
Printing a "UseLargePages is unsupported" message in the latter case
would be misleading.

I wonder if checking something like /proc/meminfo for HugePages_* is a
more robust way to probe capabilities, and also whether this is more
suited as a test harness feature, i.e., enhance jtreg and tag these tests
so that they're ignored on systems that doesn't have any/enough huge
pages.

Thanks!

/Claes

On 2018-04-22 23:18, B. Blaser wrote:
> [ I've trouble subscribing to hotspot-dev, please forward if necessary. ]
>
> Hi,
>
> After a clean build, some hotspot tests related to large page use are
> failing on my 64-bit Linux system, for example:
>
> gc/g1/TestLargePageUseForAuxMemory.java
> [...]
>
> Or simply:
>
> $ ./build/linux-x86_64-normal-server-release/images/jdk/bin/java
> -XX:+UseLargePages -version
>
> is crashing the JVM because the latter assumes that large pages are
> always supported on Linux, which appears to be wrong.
>
> I suggest to make sure that large pages are supported when parsing the
> arguments, as below.
>
> Does this look reasonable (tier1 looks better now)?
>
> Thanks,
> Bernard
>
> diff -r 8c85a1855e10 src/hotspot/share/runtime/arguments.cpp
> --- a/src/hotspot/share/runtime/arguments.cpp Fri Apr 13 11:14:49 2018 -0700
> +++ b/src/hotspot/share/runtime/arguments.cpp Sun Apr 22 20:29:21 2018 +0200
> @@ -60,6 +60,7 @@
>   #include "utilities/defaultStream.hpp"
>   #include "utilities/macros.hpp"
>   #include "utilities/stringUtils.hpp"
> +#include "sys/mman.h"
>   #if INCLUDE_JVMCI
>   #include "jvmci/jvmciRuntime.hpp"
>   #endif
> @@ -4107,6 +4108,18 @@
>     UNSUPPORTED_OPTION(UseLargePages);
>   #endif
>
> +#ifdef LINUX
> +  void *p = mmap(NULL, os::large_page_size(), PROT_READ|PROT_WRITE,
> +                 MAP_ANONYMOUS|MAP_PRIVATE|MAP_HUGETLB,
> +                 -1, 0);
> +  if (p != MAP_FAILED) {
> +    munmap(p, os::large_page_size());
> +  }
> +  else {
> +    UNSUPPORTED_OPTION(UseLargePages);
> +  }
> +#endif
> +
>     ArgumentsExt::report_unsupported_options();
>
>   #ifndef PRODUCT
> diff -r 8c85a1855e10
> test/hotspot/jtreg/runtime/memory/LargePages/TestLargePagesFlags.java
> --- a/test/hotspot/jtreg/runtime/memory/LargePages/TestLargePagesFlags.java
> Fri Apr 13 11:14:49 2018 -0700
> +++ b/test/hotspot/jtreg/runtime/memory/LargePages/TestLargePagesFlags.java
> Sun Apr 22 20:29:21 2018 +0200
> @@ -37,7 +37,7 @@
>   public class TestLargePagesFlags {
>
>     public static void main(String [] args) throws Exception {
> -    if (!Platform.isLinux()) {
> +      if (!Platform.isLinux() || !canUse(UseLargePages(true))) {
>         System.out.println("Skipping. TestLargePagesFlags has only been
> implemented for Linux.");
>         return;
>       }