Every time the OOM killer mis-aims? 🙃
Quote Tweet
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?
Show this thread
1
1
22
Conversation
Every time I want some control over what page sizes are used?
2
6
Every time I want to rearrange virtual space within a process? mremap() 🤮
3
5
Replying to
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...
1
1
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.
1
1
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.
1
1
This is my test case if you're curious:
gist.github.com/thestinger/8a1
I haven't really bothered trying to get this fixed upstream at this point. It doesn't really help me that much since it would be so long before I could assume that the kernel actually works properly.
1
1
In this test case, the issue seems to be that it tracks whether a VMA was ever PROT_WRITE and it won't merge one that was PROT_WRITE at some point with one that wasn't. Maybe that makes sense. I think my actual problem was slightly different than this test case I ended up making.
I can't clearly remember why exactly github.com/GrapheneOS/har ends up triggering a similar issue. I mean... if you look at that, it doesn't seem like it would hit a similar issue, but somehow it does.
It extends guard to add space then unprotects that amount from the start of it.
1
IIRC, the issue is that the expanded usable area between the guards doesn't get reliably merged into a single VMA. That ends up breaking the MREMAP_MAYMOVE case below since it gets EINVAL when it tries to remap the usable area. It's such a nonsense API. Really hate it.
1
Show replies