RFR: JDK-8313319: [linux] mmap should use MAP_FIXED_NOREPLACE if available [v3]

[David Holmes]

On Wed, 23 Aug 2023 09:13:13 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> 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.
>
> Thomas Stuefe has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:
> 
>  - feedback david
>  - Merge branch 'master' into JDK-8313319-MAP_FIXED_NOREPLACE
>  - Merge branch 'openjdk:master' into JDK-8313319-MAP_FIXED_NOREPLACE
>  - DK-8313319-MAP_FIXED_NOREPLACE

Testing was okay.

One typo left but otherwise seems fine to me.

Thanks

src/hotspot/os/linux/os_linux.cpp line 3494:

> 3492:   // Since Linux 4.17, the kernel offers MAP_FIXED_NOREPLACE. With this flag, if a pre-
> 3493:   // existing mapping exists, the kernel will not map at an alternative point but instead
> 3494:   // return an error. We can therefore save that unnecassary mmap-munmap cycle.

unnecessary is still mis-spelt.

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Marked as reviewed by dholmes (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/15170#pullrequestreview-1592493650
PR Review Comment: https://git.openjdk.org/jdk/pull/15170#discussion_r1303598053