Re: [PATCH v6 0/3] per-vma locks in userfaultfd

[Liam R. Howlett]

* Lokesh Gidra <lokeshgidra@xxxxxxxxxx> [240213 16:57]:
> Performing userfaultfd operations (like copy/move etc.) in critical
> section of mmap_lock (read-mode) causes significant contention on the
> lock when operations requiring the lock in write-mode are taking place
> concurrently. We can use per-vma locks instead to significantly reduce
> the contention issue.
> 
> Android runtime's Garbage Collector uses userfaultfd for concurrent
> compaction. mmap-lock contention during compaction potentially causes
> jittery experience for the user. During one such reproducible scenario,
> we observed the following improvements with this patch-set:
> 
> - Wall clock time of compaction phase came down from ~3s to <500ms
> - Uninterruptible sleep time (across all threads in the process) was
>   ~10ms (none in mmap_lock) during compaction, instead of >20s

This series looks good, Thanks!

Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>

> 
> Changes since v5 [5]:
> - Use abstract function names (like uffd_mfill_lock/uffd_mfill_unlock)
>   to avoid using too many #ifdef's, per Suren Baghdasaryan and Liam
>   Howlett
> - Use 'unlikely' (as earlier) to anon_vma related checks, per Liam Howlett
> - Eliminate redundant ptr->err->ptr conversion, per Liam Howlett
> - Use 'int' instead of 'long' for error return type, per Liam Howlett
> 
> Changes since v4 [4]:
> - Fix possible deadlock in find_and_lock_vmas() which may arise if
>   lock_vma() is used for both src and dst vmas.
> - Ensure we lock vma only once if src and dst vmas are same.
> - Fix error handling in move_pages() after successfully locking vmas.
> - Introduce helper function for finding dst vma and preparing its
>   anon_vma when done in mmap_lock critical section, per Liam Howlett.
> - Introduce helper function for finding dst and src vmas when done in
>   mmap_lock critical section.
> 
> Changes since v3 [3]:
> - Rename function names to clearly reflect which lock is being taken,
>   per Liam Howlett.
> - Have separate functions and abstractions in mm/userfaultfd.c to avoid
>   confusion around which lock is being acquired/released, per Liam Howlett.
> - Prepare anon_vma for all private vmas, anonymous or file-backed,
>   per Jann Horn.
> 
> Changes since v2 [2]:
> - Implement and use lock_vma() which uses mmap_lock critical section
>   to lock the VMA using per-vma lock if lock_vma_under_rcu() fails,
>   per Liam R. Howlett. This helps simplify the code and also avoids
>   performing the entire userfaultfd operation under mmap_lock.
> 
> Changes since v1 [1]:
> - rebase patches on 'mm-unstable' branch
> 
> [1] https://lore.kernel.org/all/20240126182647.2748949-1-lokeshgidra@xxxxxxxxxx/
> [2] https://lore.kernel.org/all/20240129193512.123145-1-lokeshgidra@xxxxxxxxxx/
> [3] https://lore.kernel.org/all/20240206010919.1109005-1-lokeshgidra@xxxxxxxxxx/
> [4] https://lore.kernel.org/all/20240208212204.2043140-1-lokeshgidra@xxxxxxxxxx/
> [5] https://lore.kernel.org/all/20240213001920.3551772-1-lokeshgidra@xxxxxxxxxx/
> 
> Lokesh Gidra (3):
>   userfaultfd: move userfaultfd_ctx struct to header file
>   userfaultfd: protect mmap_changing with rw_sem in userfaulfd_ctx
>   userfaultfd: use per-vma locks in userfaultfd operations
> 
>  fs/userfaultfd.c              |  86 ++-----
>  include/linux/userfaultfd_k.h |  75 ++++--
>  mm/userfaultfd.c              | 438 +++++++++++++++++++++++++---------
>  3 files changed, 405 insertions(+), 194 deletions(-)
> 
> -- 
> 2.43.0.687.g38aa6559b0-goog
>