On Tue, Apr 4, 2023 at 2:38 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > On Tue, 4 Apr 2023 00:13:50 +0000 Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > > > 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. > > > > Wouldn't it be better to do this as a single one-line patch for > backportability? Then all the refactoring etcetera can be added on > later. Without refactoring the code that adds reclaim_state->reclaimed to scan_control->nr_reclaimed into a helper (flush_reclaim_state()), the change would need to be done in two places instead of one, and I wouldn't know where to put the huge comment. One thing that I can do is break down patch 2 into two patches, one that adds the flush_reclaim_state() helper, and one that adds the mm_account_reclaimed_pages() helper. The series would be: Patch 1: move set_task_reclaim_state() near other helpers Patch 2: introduce mm_account_reclaimed_pages() Patch 3: introduce flush_reclaim_state() Patch 4: add the one-line change (and the huge comment) to flush_reclaim_state() Backports need only to take patches 3 & 4 (which would be localized to mm/vmscan.c), as patches 1 & 2 would be purely cosmetic with no dependency from patches 3 & 4. For the current series, patch 1 is not needed anyway. So this change would basically save backporters the part of patch 2 that is outside of mm/vmscan.c. If you think this would be useful I can send a v5 with patch 2 broken down into two patches.
