Linux Userfaultfd¶
Version History
| Date | Description |
|---|---|
| Jan 6, 2021 | Minor update |
| Jun 4, 2019 | Initial version |
Code Study¶
(Notes based on linux 5.2-rc3)
- Code Layout
- Major file:
fs/userfaultfd.c, which has all the functions and callbacks. - Callers spread across:
mm/memory.c,mm/mremap.c,mm/mmap.c, and some others. - The userfaultfd code is not that hard to understand if you already know how waitqueue etc work. It’s built center around the
file_ops, and couple callbacks for mm.
- Major file:
handle_userfault(), called bymm/memory.c:- Userfaultfd callback only happens for anonymous pgfault
- Userfaultfd skip all the LRU, rmap, cgroup
- Userfaultfd does not use the shared global zero page
userfaultfd_unmap_prep(), userfaultfd_unmap_complete(), called bymm/mmap.c, andmm/mremap.c:- Userfaultfd got notified if there are remap and unmap
- Userfaultfd deliver events via
userfaultfd_event_wait_completion() - I found code in mmap.c and mremap.c is NOT skipping rmap/lru code. Since userfaultfd related pages don’t have these setup during pgfault, I think those rmap/lru cleanup code will notice this and handle it well. In conclusion, userfault skip the expansive rmap/lru setup/teardown.
Why userfaultfd?¶
It was at first developed to enhance VM migration: after migration, the destination QEMU can handle pgfault and bring pages from remote via network.
Some databases also use it to have customized feature: http://tech.adroll.com/blog/data/2016/11/29/traildb-mmap-s3.html. Some academic papers are also using it to do customized processing in user space (e.g., remote regions). But I don’t think this is going to be practical for performance-critical systems.
My thought? The use case is very similar to what we did in Hotpot: get the faulting user address, and fetch it from remote. Due to kernel limitations and security constraints, the userfaultfd has to go through many layers and multiple kernel/user crossing. It would be interesting to inject eBPF code from user to kernel to handle pgfault (any research value?)