Every time the OOM killer mis-aims? 🙃
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In a way you never do with Linux. Because its design is generic. It’s meant to run programs for you. But when’s the last time it’s memory model got in your way?
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Every time I want some control over what page sizes are used?
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Every time I want to rearrange virtual space within a process? mremap() 🤮
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mremap is super broken because it returns EINVAL if you try to use it on multiple VMAs but the kernel does not reliably merge compatible VMAs.
I have a simple test case involving switching to PROT_NONE and back where it fails to merge them, and then mremap will return EINVAL...
Look at how painful it was to write this code to remap with guard pages:
github.com/GrapheneOS/har
MREMAP_DONTUNMAP can theoretically be used but I don't think it can expand the size of the mapping so it will be necessary to use mprotect in a way that may lead to non-merged VMAs.
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It's the reason this in-place growth code was disabled:
github.com/GrapheneOS/har
It isn't particularly useful anyway, because in-place growth rarely works in practice.
There's separate code above that for in-place shrink.
All of this is needed due to the limitation of 1 VMA.
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