Recently, we have observed some janky issues caused by unpleasantly long contention on mmap_lock which is held by smaps_rollup when probing large processes. To address the problem, we let smaps_rollup detect if anyone wants to acquire mmap_lock for write attempts. If yes, just release the lock temporarily to ease the contention. smaps_rollup is a procfs interface which allows users to summarize the process's memory usage without the overhead of seq_* calls. Android uses it to sample the memory usage of various processes to balance its memory pool sizes. If no one wants to take the lock for write requests, smaps_rollup with this patch will behave like the original one. Although there are on-going mmap_lock optimizations like range-based locks, the lock applied to smaps_rollup would be the coarse one, which is hard to avoid the occurrence of aforementioned issues. So the detection and temporary release for write attempts on mmap_lock in smaps_rollup is still necessary. Change since v1: - If current VMA is freed after dropping the lock, it will return - incomplete result. To fix this issue, refine the code flow as - suggested by Steve. [1] Change since v2: - When getting back the mmap lock, the address where you stopped last - time could now be in the middle of a vma. Add one more check to handle - this case as suggested by Michel. [2] Change since v3: - last_stopped is easily confused with last_vma_end. Replace it with - a direct call to smap_gather_stats(vma, &mss, last_vma_end) as - suggested by Steve. [3] [1] https://lore.kernel.org/lkml/bf40676e-b14b-44cd-75ce-419c70194783@xxxxxxx/ [2] https://lore.kernel.org/lkml/CANN689FtCsC71cjAjs0GPspOhgo_HRj+diWsoU1wr98YPktgWg@xxxxxxxxxxxxxx/ [3] https://lore.kernel.org/lkml/db0d40e2-72f3-09d5-c162-9c49218f128f@xxxxxxx/ Chinwen Chang (3): mmap locking API: add mmap_lock_is_contended() mm: smaps*: extend smap_gather_stats to support specified beginning mm: proc: smaps_rollup: do not stall write attempts on mmap_lock fs/proc/task_mmu.c | 96 +++++++++++++++++++++++++++++++++++---- include/linux/mmap_lock.h | 5 ++ 2 files changed, 92 insertions(+), 9 deletions(-)
