8325108: POSIX map_memory_to_file calls release_memory unnecessarily #29081

jdksjolen · 2026-01-07T09:16:57Z

The implementation of the function os::map_memory_to_file calls os::release_memory if a mmap:ing to a fixed location succeeds but is not at the requested location. A POSIX implementation of mmap is not allowed to succeed and return a different base address if MAP_FIXED is supplied, so this is dead code. We can remove it.

Quote:

When MAP_FIXED is set in the flags argument, the implementation is informed that the value of pa shall be addr, exactly. If MAP_FIXED is set, mmap() may return MAP_FAILED and set errno to [EINVAL]. If a MAP_FIXED request is successful, the mapping established by mmap() replaces any previous mappings for the process' pages in the range [pa,pa+len).

https://pubs.opengroup.org/onlinepubs/009604499/functions/mmap.html

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JDK-8325108: POSIX map_memory_to_file calls release_memory unnecessarily (Bug - P4)

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David Holmes (@dholmes-ora - Reviewer)
Coleen Phillimore (@coleenp - Reviewer)

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bridgekeeper · 2026-01-07T09:18:52Z

👋 Welcome back jsjolen! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2026-01-07T09:19:43Z

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

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8325108: POSIX map_memory_to_file calls release_memory unnecessarily

Reviewed-by: dholmes, coleenp

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At the time when this comment was updated there had been 200 new commits pushed to the master branch:

385c4f8: 8373714: Shenandoah: Register heuristic penalties following a degenerated GC
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1342db0: 8374051: Incorrect parameterized testing of exceptions in AbstractDateTimeTest.java
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openjdk · 2026-01-07T09:20:51Z

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

hotspot-runtime

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.

mlbridge · 2026-01-08T11:21:46Z

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01: Full - Incremental (55ffac69)
00: Full (f5bf7141)

coleenp · 2026-01-08T18:12:47Z

src/hotspot/os/posix/os_posix.cpp

    warning("Failed mmap to file. (%s)", os::strerror(errno));
    return nullptr;
  }
-  if (base != nullptr && addr != base) {


Can you turn this into an assert with a comment in the assert string why this isn't possible?

I like the assert!

dholmes-ora

Dead code removal seems fine. Thanks

coleenp

Looks good!

coleenp · 2026-01-08T23:17:44Z

src/hotspot/os/posix/os_posix.cpp

    warning("Failed mmap to file. (%s)", os::strerror(errno));
    return nullptr;
  }
-  if (base != nullptr && addr != base) {


I like the assert!

Remove dead branch

f5bf714

openjdk bot changed the title ~~8361382~~ 8361382: NMT corruption: header canary broken as part of CodeBlob::purge() Jan 7, 2026

openjdk bot added the hotspot-runtime hotspot-runtime-dev@openjdk.org label Jan 7, 2026

jdksjolen changed the title ~~8361382: NMT corruption: header canary broken as part of CodeBlob::purge()~~ 8325108 Jan 7, 2026

openjdk bot changed the title ~~8325108~~ 8325108: POSIX map_memory_to_file calls release_memory unnecessarily Jan 7, 2026

jdksjolen marked this pull request as ready for review January 8, 2026 11:17

openjdk bot added the rfr Pull request is ready for review label Jan 8, 2026

coleenp reviewed Jan 8, 2026

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Check for expected invariant

55ffac6

dholmes-ora approved these changes Jan 8, 2026

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coleenp approved these changes Jan 8, 2026

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