The issue this patchset is trying to address is mmap_lock contention when a low priority task (monitoring, data collecting, etc.) blocks a higher priority task from making updated to the address space. The contention is due to the mmap_lock being held for read when reading proc/pid/maps. With maple_tree introduction, VMA tree traversals are RCU-safe and per-vma locks make VMA access RCU-safe. this provides an opportunity for lock-less reading of proc/pid/maps. We still need to overcome a couple obstacles: 1. Make all VMA pointer fields used for proc/pid/maps content generation RCU-safe; 2. Ensure that proc/pid/maps data tearing, which is currently possible at page boundaries only, does not get worse. The patchset deals with these issues but there is a downside which I would like to get input on: This change introduces unfairness towards the reader of proc/pid/maps, which can be blocked by an overly active/malicious address space modifyer. A couple of ways I though we can address this issue are: 1. After several lock-less retries (or some time limit) to fall back to taking mmap_lock. 2. Employ lock-less reading only if the reader has low priority, indicating that blocking it is not critical. 3. Introducing a separate procfs file which publishes the same data in lock-less manner. I imagine a combination of these approaches can also be employed. I would like to get feedback on this from the Linux community. Note: mmap_read_lock/mmap_read_unlock sequence inside validate_map() can be replaced with more efficiend rwsem_wait() proposed by Matthew in [1]. [1] https://lore.kernel.org/all/ZZ1+ZicgN8dZ3zj3@xxxxxxxxxxxxxxxxxxxx/ Suren Baghdasaryan (3): mm: make vm_area_struct anon_name field RCU-safe seq_file: add validate() operation to seq_operations mm/maps: read proc/pid/maps under RCU fs/proc/internal.h | 3 + fs/proc/task_mmu.c | 130 ++++++++++++++++++++++++++++++++++---- fs/seq_file.c | 24 ++++++- include/linux/mm_inline.h | 10 ++- include/linux/mm_types.h | 3 +- include/linux/seq_file.h | 1 + mm/madvise.c | 30 +++++++-- 7 files changed, 181 insertions(+), 20 deletions(-) -- 2.43.0.381.gb435a96ce8-goog
