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JDK-8313319: [linux] mmap should use MAP_FIXED_NOREPLACE if available #15170

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tstuefe
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@tstuefe tstuefe commented Aug 6, 2023

os::attempt_reserve_memory_at() is used to map memory at a specific address that we fancy. It is not used to replace an existing mapping (hence we omit MAP_FIXED).

But it may result in the kernel creating a mapping at a different address if our wish address is blocked; we then have to unmap again. That mmap-munmap cycle is unnecessary.

MAP_FIXED_NOREPLACE exists since Linux 4.17 and prevents that unnecessary cycle . We should use that if available. We need to tiptoe around a bit to deal with building on old machines/running on old kernels, though.


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  • JDK-8313319: [linux] mmap should use MAP_FIXED_NOREPLACE if available (Enhancement - P4)

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@tstuefe The following label will be automatically applied to this pull request:

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When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.

@openjdk openjdk bot added the hotspot-runtime hotspot-runtime-dev@openjdk.org label Aug 6, 2023
@tstuefe tstuefe force-pushed the JDK-8313319-MAP_FIXED_NOREPLACE branch 2 times, most recently from 9276e14 to 4f9eec8 Compare August 7, 2023 18:08
@tstuefe tstuefe force-pushed the JDK-8313319-MAP_FIXED_NOREPLACE branch from 4f9eec8 to bd17fe2 Compare August 7, 2023 18:10
@tstuefe tstuefe marked this pull request as ready for review August 8, 2023 05:52
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tstuefe commented Aug 8, 2023

Windows x64 GHA errors unrelated

@openjdk openjdk bot added the rfr Pull request is ready for review label Aug 8, 2023
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Hi,

Thanks for this and looks good to me. Two small nits.

Comment on lines +3496 to +3497
// Backward compatibility: Older kernels will ignore the unknown flag; so mmap will behave
// as in mode (a).
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Is this in case of a libc that's more up to date than the kernel?

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The libc has little to do with it. mmap is a system call, and the flags should come from mman.h, which should be a kernel header.

But I am not sure whether the flags we use when including mman.h are actually directly from the kernel, because the glibc contains the same flags and talks about "kernel header is not namespace clean", whatever that means.

Bottomline, on the build machine we either have the flag - from libc or kernel - or we don't, in which case we define it ourselves. At runtime, the kernel either understands the flag or it don't, in the latter case it shears the flag off and then behaves as if the flag had not been given.

See also man 2 mmap:

Note that older kernels which do not recognize the MAP_FIXED_NOREPLACE flag will typically (upon detecting a collision with a preexisting mapping) fall back to a "non-MAP_FIXED" type of behavior: they will return an address that is different from the requested address. Therefore, backward-compatible software should check the returned address against the requested address.

ASSERT_EQ(p2, (char*)nullptr); // should have failed
os::release_memory(p1, M);
} else {
tty->print_cr("Skipped.");
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Have you succesfully seen this printed? AFAIK Gtest only prints stderr by default

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Yes, it gets printed.

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openjdk bot commented Aug 8, 2023

@tstuefe This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8313319: [linux] mmap should use MAP_FIXED_NOREPLACE if available

Reviewed-by: jsjolen, dholmes

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 16 new commits pushed to the master branch:

  • 3285a1e: 8314555: Build with mawk fails on Windows
  • 4a50e87: 8303852: current_stack_region() gets called twice unnecessarily
  • c077be4: 8314694: Separate checked_cast from globalDefinitions.hpp
  • 68815d5: 8314734: Remove unused field TypeVariableImpl.EMPTY_ANNOTATION_ARRAY
  • 57a322d: 8308042: [macos] Developer ID Application Certificate not picked up by jpackage if it contains UNICODE characters
  • 38a9edf: 8314679: SA fails to properly attach to JVM after having just detached from a different JVM
  • 2c60cad: 8280743: HSDB "Monitor Cache Dump" command might throw NPE
  • 9435cd1: 8175874: Update Security.insertProviderAt to specify behavior when requested position is out of range.
  • dbb788f: 8294535: Add screen capture functionality to PassFailJFrame
  • fae3b02: 8314746: Remove unused private put* methods from DirectByteBufferR
  • ... and 6 more: https://git.openjdk.org/jdk/compare/2be469f89ec10471e893045e606e1e2558f4e363...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

➡️ To integrate this PR with the above commit message to the master branch, type /integrate in a new comment.

@openjdk openjdk bot added the ready Pull request is ready to be integrated label Aug 8, 2023
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tstuefe commented Aug 8, 2023

Hi,

Thanks for this and looks good to me. Two small nits.

Thanks Johan!

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tstuefe commented Aug 14, 2023

Friendly ping.

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tstuefe commented Aug 22, 2023

Anybody else? Need a second review. Thanks!

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This seems reasonable. I'd like to take it for a spin through our CI though. Thanks.

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tstuefe commented Aug 23, 2023

This seems reasonable. I'd like to take it for a spin through our CI though. Thanks.

Thank you, David!

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Testing was okay.

One typo left but otherwise seems fine to me.

Thanks

// to map there and nowhere else. Therefore we will unmap the block again, which means we
// just executed a needless mmap->munmap cycle.
// Since Linux 4.17, the kernel offers MAP_FIXED_NOREPLACE. With this flag, if a pre-
// existing mapping exists, the kernel will not map at an alternative point but instead
// return an error. We can therefore save that unnessassary mmap-munmap cycle.
// return an error. We can therefore save that unnecassary mmap-munmap cycle.
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unnecessary is still mis-spelt.

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Thank you, David, for review and testing. I fixed the typo.

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tstuefe commented Aug 24, 2023

/integrate

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openjdk bot commented Aug 24, 2023

Going to push as commit 3699666.
Since your change was applied there have been 16 commits pushed to the master branch:

  • 3285a1e: 8314555: Build with mawk fails on Windows
  • 4a50e87: 8303852: current_stack_region() gets called twice unnecessarily
  • c077be4: 8314694: Separate checked_cast from globalDefinitions.hpp
  • 68815d5: 8314734: Remove unused field TypeVariableImpl.EMPTY_ANNOTATION_ARRAY
  • 57a322d: 8308042: [macos] Developer ID Application Certificate not picked up by jpackage if it contains UNICODE characters
  • 38a9edf: 8314679: SA fails to properly attach to JVM after having just detached from a different JVM
  • 2c60cad: 8280743: HSDB "Monitor Cache Dump" command might throw NPE
  • 9435cd1: 8175874: Update Security.insertProviderAt to specify behavior when requested position is out of range.
  • dbb788f: 8294535: Add screen capture functionality to PassFailJFrame
  • fae3b02: 8314746: Remove unused private put* methods from DirectByteBufferR
  • ... and 6 more: https://git.openjdk.org/jdk/compare/2be469f89ec10471e893045e606e1e2558f4e363...master

Your commit was automatically rebased without conflicts.

@openjdk openjdk bot added the integrated Pull request has been integrated label Aug 24, 2023
@openjdk openjdk bot closed this Aug 24, 2023
@openjdk openjdk bot removed the ready Pull request is ready to be integrated label Aug 24, 2023
@openjdk openjdk bot removed the rfr Pull request is ready for review label Aug 24, 2023
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@tstuefe Pushed as commit 3699666.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

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