* 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 >
