Upon running some proactive reclaim tests using memory.reclaim, we noticed some tests flaking where writing to memory.reclaim would be successful even though we did not reclaim the requested amount fully. Looking further into it, I discovered that *sometimes* we over-report the number of reclaimed pages in memcg reclaim. Reclaimed pages through other means than LRU-based reclaim are tracked through reclaim_state in struct scan_control, which is stashed in current task_struct. These pages are added to the number of reclaimed pages through LRUs. For memcg reclaim, these pages generally cannot be linked to the memcg under reclaim and can cause an overestimated count of reclaimed pages. This short series tries to address that. Patches 1-2 are just refactoring, they add helpers that wrap some operations on current->reclaim_state, and rename reclaim_state->reclaimed_slab to reclaim_state->reclaimed. Patch 3 ignores pages reclaimed outside of LRU reclaim in memcg reclaim. The pages are uncharged anyway, so even if we end up under-reporting reclaimed pages we will still succeed in making progress during charging. Do not let the diff stat deceive you, the core of this series is patch 3, which has one line of code change. All the rest is refactoring and one huge comment. v1 -> v2: - Renamed report_freed_pages() to mm_account_reclaimed_pages(), as suggested by Dave Chinner. There were discussions about leaving updating current->reclaim_state open-coded as it's not worth hiding the current dereferencing to remove one line, but I'd rather have the logic contained with mm/vmscan.c so that the next person that changes this logic doesn't have to change 7 different files. - Renamed add_non_vmscan_reclaimed() to flush_reclaim_state() (Johannes Weiner). - Added more context about how this problem was found in the cover letter (Johannes Weiner). - Added a patch to move set_task_reclaim_state() below the definition of cgroup_reclaim(), and added additional helpers in the same position. This way all the helpers for reclaim_state live together, and there is no need to declare cgroup_reclaim() early or move its definition around to call it from flush_reclaim_state(). This should also fix the build error reported by the bot in !CONFIG_MEMCG. RFC -> v1: - Exported report_freed_pages() in case XFS is built as a module (Matthew Wilcox). - Renamed reclaimed_slab to reclaim in previously missed MGLRU code. - Refactored using reclaim_state to update sc->nr_reclaimed into a helper and added an XL comment explaining why we ignore reclaim_state->reclaimed in memcg reclaim (Johannes Weiner). Yosry Ahmed (3): mm: vmscan: move set_task_reclaim_state() after cgroup_reclaim() mm: vmscan: refactor updating reclaimed pages in reclaim_state mm: vmscan: ignore non-LRU-based reclaim in memcg reclaim fs/inode.c | 3 +- fs/xfs/xfs_buf.c | 3 +- include/linux/swap.h | 5 ++- mm/slab.c | 3 +- mm/slob.c | 6 +-- mm/slub.c | 5 +-- mm/vmscan.c | 88 +++++++++++++++++++++++++++++++++++--------- 7 files changed, 81 insertions(+), 32 deletions(-) -- 2.40.0.rc0.216.gc4246ad0f0-goog
