Re: [PATCH v4 0/5] mm: restore per-memcg reclaim for NONSLAB shrinkers under nokmem

[Zi Yan]

On 26 Oct 2026, at 2:26, Qinyun Tan wrote:

> Hi, All
>
> Gentle ping on this series.
>
> V4 fixes the boot hang Kairui reported against v3 -- the root cause
> was that the walker reverse-resolved xarray keys as private memcg IDs
> after the keys were switched to cgroup IDs, causing shrink_dcache_sb()
> to loop forever on the global drain. The fix passes the xarray index
> directly through the internal walk path (new patch 1), so no reverse
> lookup is needed.
>
> Could someone take a look when they have a chance?

People are currently busy with reviewing various mm fixes[1][2].
Hopefully we will come back to this after the merge window closes.

[1] https://lore.kernel.org/all/20261027-fixes_core-v1-0-6f74bec9a5bb@xxxxxxxxxx/
[2] https://lore.kernel.org/all/20261027-fixes_shmem-v2-0-86a00952acb9@xxxxxxxxxx/

>
> Thanks,
> Qinyun Tan
>
> On 10/13/26 11:12 AM, Qinyun Tan wrote:
>> With cgroup.memory=nokmem, the THP deferred split shrinker and the
>> zswap shrinker are degraded in two ways.
>>
>> First, both shrinkers are missing the SHRINKER_NONSLAB flag, so
>> shrinker_memcg_alloc() demotes them to non-memcg-aware shrinkers:
>> limit-induced reclaim of a cgroup neither splits its partially
>> unmapped THPs nor writes back its zswapped pages.  v1 of this series
>> [1] restored the flag to fix that.
>>
>> However, as Sashiko's review of v1 pointed out [2], the flag alone
>> is not enough.  __list_lru_init() also collapses every list_lru into
>> per-node lists under nokmem, so even with the flag restored, the
>> objects of all cgroups share one list per node: the per-memcg
>> shrinker bit is only set for whichever memcg happens to repopulate
>> the empty list, so pressure in other cgroups may not even trigger
>> the scan, and when it does, the scan walks everyone's objects.
>>
>> Before commit fafaeceb89a5 ("mm: switch deferred split shrinker to
>> list_lru"), THP had fully per-memcg deferred split queues embedded
>> in struct mem_cgroup, working independently of kmem accounting.
>> nokmem only opts out of kernel slab accounting; THPs and zswapped
>> pages are user memory and remain charged to their cgroups, so
>> per-memcg reclaim is still what these shrinkers want.
>>
>> This series keeps list_lrus backed by SHRINKER_NONSLAB shrinkers
>> memcg aware under nokmem:
>>
>> Patch 1 passes the xarray index directly to the list_lru walker and
>> drops the reverse memcg lookup that v3 still performed between the
>> xarray iteration and the walk.  That lookup was the cause of the
>> boot hang reported against v3 [3]: after the keys were switched to
>> cgroup IDs, the walker kept resolving them as private memcg IDs, so
>> the global drain kept skipping non-empty per-memcg sublists and
>> shrink_dcache_sb() could loop forever.  Passing the index directly
>> also removes the temporary css reference from the walk.
>>
>> Patch 2 drops the kmemcg_id copy, which is only assigned when kmem
>> accounting is enabled, and indexes the list_lru xarray with
>> mem_cgroup_id() instead, so the index works independently of kmem
>> accounting.  The list_lru plumbing passes memcg pointers now, and
>> memcg_online_kmem()/memcg_offline_kmem() are inlined into the css
>> online/offline hooks.  It also drops the nokmem early return from
>> the offline path so these lrus are reparented on offline.
>>
>> Patch 3 keeps a list_lru memcg aware under nokmem when its backing
>> shrinker is registered SHRINKER_NONSLAB.
>>
>> Patches 4 and 5 restore/add SHRINKER_NONSLAB on the THP deferred
>> split shrinker and the zswap shrinker.
>>
>> The savings of nokmem are preserved: slab-backed lrus (e.g. the
>> superblock dentry/inode lrus) still fall back to per-node lists, and
>> the per-memcg lists are only allocated when a memcg actually holds
>> such objects.
>>
>> Changes in v4:
>>  - Fix the boot hang reported by Kairui against v3 [3]: the walker
>>    no longer reverse-resolves the xarray keys; the index is passed
>>    directly through the internal walk path (new patch 1, split out
>>    as a preparation so patch 2 remains a pure key switch).
>>  - Drop the kmemcg_id member from memcg_struct_check() along with
>>    the field removal, as pointed out by Gregory Price [4].
>>  - Otherwise unchanged from v3.
>>
>> Changes in v3:
>>  - Index the list_lru xarray with mem_cgroup_id() (the cgroup ID)
>>    instead of the private memcg ID, whose lifetime only covers
>>    online groups, and pass memcg pointers through the list_lru
>>    plumbing; inline memcg_online_kmem()/memcg_offline_kmem() into
>>    the css online/offline hooks (patch 1 as numbered in v3, per
>>    Johannes's review of v2 [5]).
>>  - Describe in the THP patch's changelog why fafaeceb89a5 dropped
>>    SHRINKER_NONSLAB knowingly, per Johannes's pointer to the
>>    original discussion [5].
>>  - Collected review tags: Johannes's Reviewed-by on patches 2-4,
>>    Nhat's and Yosry's Acked-by on patch 4 (numbering as in v3).
>>
>> Changes in v2:
>>  - Keep the list_lrus behind the two shrinkers per-memcg under
>>    nokmem, so the shrinker bits are set for the right memcgs and
>>    scans stay scoped to the target cgroup's objects (patches 1-2
>>    as numbered in v2, new; addresses Sashiko's review of v1 [2]).
>>  - Patches 3-4 unchanged from v1; collected the review tags.
>>
>> [1] https://lore.kernel.org/lkml/20260904033503.4067283-1-qinyuntan@xxxxxxxxxxxxxxxxx/
>> [2] https://sashiko.dev/#/patchset/20260904033503.4067283-1-qinyuntan@xxxxxxxxxxxxxxxxx
>> [3] https://lore.kernel.org/lkml/20260910080722.3961351-1-qinyuntan@xxxxxxxxxxxxxxxxx/
>> [4] https://lore.kernel.org/lkml/arNK64PxTXbVEhis@gourry-fedora-PF4VCD3F/
>> [5] https://lore.kernel.org/lkml/20260907110111.2286932-1-qinyuntan@xxxxxxxxxxxxxxxxx/
>>
>> Qinyun Tan (5):
>>   mm: list_lru: pass the memcg index directly to the walker
>>   mm: memcontrol: drop kmemcg_id and use mem_cgroup_id() for list_lru
>>     indexing
>>   mm: list_lru: keep per-memcg lists with nokmem for NONSLAB-backed lrus
>>   mm: thp: restore SHRINKER_NONSLAB on the deferred split shrinker
>>   mm: zswap: mark the zswap shrinker SHRINKER_NONSLAB
>>
>>  include/linux/memcontrol.h |  16 ------
>>  mm/huge_memory.c           |   3 +-
>>  mm/list_lru.c             | 114 +++++++++++++++++++++----------------
>>  mm/memcontrol.c            |  45 +++------------
>>  mm/zswap.c                 |   4 +-
>>  5 files changed, 77 insertions(+), 105 deletions(-)
>>


Best Regards,
Yan, Zi