[PATCH mm-unstable v5 0/2] mm/vmscan: fix NR_ISOLATED accounting and throttle MGLRU eviction

[Hui Zhu]

From: Hui Zhu <zhuhui@xxxxxxxxxx>

The legacy reclaim path has two mechanisms around isolated folios that
MGLRU lacks:

1. NR_ISOLATED_ANON/FILE counters are updated when folios are isolated
   from the inactive lists. Compaction's too_many_isolated() relies on
   them to decide when to back off. The MGLRU reclaim path never
   updates them, so compaction cannot see MGLRU's in-flight isolation.

2. shrink_inactive_list() throttles direct reclaim via
   too_many_isolated() when isolated folios pile up. MGLRU's
   evict_folios() isolates folios without any such check, so many
   concurrent reclaimers can over-isolate the same (oldest) generation,
   leading to unnecessary swapping, thrashing and premature memcg OOM.

Patch 1 fixes the counter accounting in the MGLRU isolation path.
Patch 2 adds a per-lruvec throttle, mirroring the legacy behavior but
adapted to MGLRU's per-lruvec contention and its dynamic type
selection/fallback.

Testing
=======
Two test scripts are provided to reproduce the problem and validate
the fix. Both are available at:
https://gist.github.com/teawater/3ef51251f2e91a5a600e3d26bb477e34
Test environment: 10 CPU / 8GB QEMU guest, MGLRU enabled, a 16MB
memory cgroup, anonymous working set, swap backed by dm-delay (50ms
read/write delay) to slow swap-out and lengthen the isolation window.

mglru_iso_repro.sh (64 threads, 48MB working set, 60s):
Drives concurrent direct reclaim inside the memcg and measures scan
efficiency, throttle events, in-flight isolation and throughput.
Neither kernel OOMs at this concurrency; the value of the patch shows
in reclaim quality:
                        unpatched       patched
  OOM kills             0               0
  mm_vmscan_throttled   0               35645 (all
                                        VMSCAN_THROTTLE_ISOLATED)
  nr_isolated peak      0 (invisible)   230
  total touches         246,499,132,369 301,456,426,692
  scan efficiency       0.0261          0.0194

The patched kernel completes ~22% more work in the same 60s: the
throttle keeps concurrent reclaimers from trampling the same
generation, so less CPU is burned in reclaim. Note nr_isolated is
always 0 on the unpatched kernel - the over-isolation is invisible
there, which is exactly what patch 1 fixes.

mglru_iso_repro_v2.sh (192 threads, 48MB working set, 60s):
Raises concurrency to the point where over-isolation becomes fatal:
                        unpatched       patched
  OOM kills             1 (task killed) 0
  memcg oom events      51              0
  mm_vmscan_throttled   0               512292 (all
                                        VMSCAN_THROTTLE_ISOLATED)
  total touches         0 (killed)      682,300,003,972

With 192 threads the unpatched kernel cannot keep reclaim ahead of
allocation and the task is OOM-killed; the patched kernel survives the
full run and keeps reclaim making progress.

Changelog:
v5:
According to the comments of Kairun and feedback from the test scripts,
make the throttle check per lruvec (new nr_isolated counter) and skip
empty types in the allowed mask.
v4:
According to the commens of Baolin and Barry, rework patch 2:
drop the throttle_is_throttled() helper extracted
from shrink_inactive_list() and leave the legacy path untouched.
The new MGLRU-only throttle_evictable_types() only sleeps when all
evictable types are over-isolated - v3 throttled as soon as any of
them was, which unnecessarily blocked the reclaim of the other type
and passes the mask of the remaining types to isolate_folios(),
which restricts both its initial choice and its fallback, so that
isolation never lands on a throttled type.  The v3 gate did not
constrain the type actually isolated, so the fallback could still
pick the over-isolated one.
Re-run the tests and update the test log.
v3:
According to the commens of Baolin, remove the redundant nr_isolated
check before restoring the NR_ISOLATED_* counters in evict_folios().
rename the extracted helper to throttle_is_throttled() to avoid
confusion with the existing wake_throttle_isolated() naming space.
Use for_each_evictable_type() in the MGLRU throttle to check each
evictable type's isolation instead of only the type returned by
get_type_to_scan(), since isolate_folios() may fall back to the
other type.
Re-run the tests and update the test log.
v2:
According to the commens of Kairun, Rebased on mm-unstable.
Split into two patches; patch 2 is new and adds the
too_many_isolated() throttling to the MGLRU eviction path, which v1
did not cover.
Add test infomations.

Hui Zhu (2):
  mm/vmscan: fix missing NR_ISOLATED counter update in MGLRU reclaim
    path
  mm/vmscan: throttle MGLRU eviction when isolated folios pile up

 include/linux/mmzone.h |   2 +
 mm/vmscan.c            | 173 +++++++++++++++++++++++++++++++++++++----
 2 files changed, 162 insertions(+), 13 deletions(-)

-- 
2.53.0