config: enabling madvise syscalls for user mode deallocations#133
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benhillis merged 1 commit intoMay 20, 2026
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Pull request overview
This PR enables CONFIG_ADVISE_SYSCALLS in the OpenHCL kernel configs so madvise() (used by MiMalloc to decommit memory) is available, allowing physical pages to be reclaimed and preventing memory growth from decommit operations being ignored.
Changes:
- Enable
CONFIG_ADVISE_SYSCALLS=yfor the x64 HCL kernel config. - Enable
CONFIG_ADVISE_SYSCALLS=yfor the arm64 HCL kernel config.
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| File | Description |
|---|---|
| Microsoft/hcl-x64.config | Enables ADVISE_SYSCALLS so MiMalloc’s madvise()-based decommit can free memory on x64. |
| Microsoft/hcl-arm64.config | Enables ADVISE_SYSCALLS so MiMalloc’s madvise()-based decommit can free memory on arm64. |
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Can you also submit a similar fix to the 6.18 branch please? |
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Yes, here is the PR for 6.18: PR 134 |
smalis-msft
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benhillis
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Can we please add the details of changes in commit message
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I have updated the commit subject and message with more substantial information on the changes. |
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This config change includes madvise.o in the final build image, enabling madvise syscalls. This is necessary because MiMalloc (the allocator for OpenHCL) uses madvise syscalls to deallocate memory. Without this change, madvise is excluded from the final build image resulting in the syscalls for deallocating memory from the user mode being unrecognized by the kernel. This results in a major memory leak where no user mode process is able to free physical memory. OpenHCL builds with the kernel with this fix indicate steady memory usage in a reproduction scenario that would otherwise expose the memory leak.
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This change updates the OpenHCL kernel to the latest version which contains this patch: microsoft/OHCL-Linux-Kernel#133 Huge props to @dpaliulis-msft for debugging this! Co-authored-by: Ben Hillis <benhill@ntdev.microsoft.com>
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MiMalloc, the allocator for OpenHCL, uses madvise syscalls to decommit memory. With madvise disabled in the kernel, madvise never triggers, causing the physical memory to not be cleaned on a decommit. This results in a memory leak. This change enables madvise syscalls, allowing memory to be properly freed when decommits are invoked by MiMalloc. Lab tests show that this change introduces some initial memory overhead, but resolves the issue of memory growth with the memory leak repro.