Re: [PATCH RFC 00/28] Eliminate Dying Memory Cgroup

[Muchun Song]

> On Apr 15, 2025, at 10:45, Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
> 
> This patchset is based on v6.15-rc2. It functions correctly only when
> CONFIG_LRU_GEN (Multi-Gen LRU) is disabled. Several issues were encountered
> during rebasing onto the latest code. For more details and assistance, refer
> to the "Challenges" section. This is the reason for adding the RFC tag.

Sorry, I forgot to CC Yu Zhao. Now I've included him. I think he may
offer useful value in this aspect.

Muchun,
Thanks.

> 
> ## Introduction
> 
> This patchset is intended to transfer the LRU pages to the object cgroup
> without holding a reference to the original memory cgroup in order to
> address the issue of the dying memory cgroup. A consensus has already been
> reached regarding this approach recently [1].
> 
> ## Background
> 
> The issue of a dying memory cgroup refers to a situation where a memory
> cgroup is no longer being used by users, but memory (the metadata
> associated with memory cgroups) remains allocated to it. This situation
> may potentially result in memory leaks or inefficiencies in memory
> reclamation and has persisted as an issue for several years. Any memory
> allocation that endures longer than the lifespan (from the users'
> perspective) of a memory cgroup can lead to the issue of dying memory
> cgroup. We have exerted greater efforts to tackle this problem by
> introducing the infrastructure of object cgroup [2].
> 
> Presently, numerous types of objects (slab objects, non-slab kernel
> allocations, per-CPU objects) are charged to the object cgroup without
> holding a reference to the original memory cgroup. The final allocations
> for LRU pages (anonymous pages and file pages) are charged at allocation
> time and continues to hold a reference to the original memory cgroup
> until reclaimed.
> 
> File pages are more complex than anonymous pages as they can be shared
> among different memory cgroups and may persist beyond the lifespan of
> the memory cgroup. The long-term pinning of file pages to memory cgroups
> is a widespread issue that causes recurring problems in practical
> scenarios [3]. File pages remain unreclaimed for extended periods.
> Additionally, they are accessed by successive instances (second, third,
> fourth, etc.) of the same job, which is restarted into a new cgroup each
> time. As a result, unreclaimable dying memory cgroups accumulate,
> leading to memory wastage and significantly reducing the efficiency
> of page reclamation.
> 
> ## Fundamentals
> 
> A folio will no longer pin its corresponding memory cgroup. It is necessary
> to ensure that the memory cgroup or the lruvec associated with the memory
> cgroup is not released when a user obtains a pointer to the memory cgroup
> or lruvec returned by folio_memcg() or folio_lruvec(). Users are required
> to hold the RCU read lock or acquire a reference to the memory cgroup
> associated with the folio to prevent its release if they are not concerned
> about the binding stability between the folio and its corresponding memory
> cgroup. However, some users of folio_lruvec() (i.e., the lruvec lock)
> desire a stable binding between the folio and its corresponding memory
> cgroup. An approach is needed to ensure the stability of the binding while
> the lruvec lock is held, and to detect the situation of holding the
> incorrect lruvec lock when there is a race condition during memory cgroup
> reparenting. The following four steps are taken to achieve these goals.
> 
> 1. The first step  to be taken is to identify all users of both functions
>   (folio_memcg() and folio_lruvec()) who are not concerned about binding
>   stability and implement appropriate measures (such as holding a RCU read
>   lock or temporarily obtaining a reference to the memory cgroup for a
>   brief period) to prevent the release of the memory cgroup.
> 
> 2. Secondly, the following refactoring of folio_lruvec_lock() demonstrates
>   how to ensure the binding stability from the user's perspective of
>   folio_lruvec().
> 
>   struct lruvec *folio_lruvec_lock(struct folio *folio)
>   {
>           struct lruvec *lruvec;
> 
>           rcu_read_lock();
>   retry:
>           lruvec = folio_lruvec(folio);
>           spin_lock(&lruvec->lru_lock);
>           if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio))) {
>                   spin_unlock(&lruvec->lru_lock);
>                   goto retry;
>           }
> 
>           return lruvec;
>   }
> 
>   From the perspective of memory cgroup removal, the entire reparenting
>   process (altering the binding relationship between folio and its memory
>   cgroup and moving the LRU lists to its parental memory cgroup) should be
>   carried out under both the lruvec lock of the memory cgroup being removed
>   and the lruvec lock of its parent.
> 
> 3. Thirdly, another lock that requires the same approach is the split-queue
>   lock of THP.
> 
> 4. Finally, transfer the LRU pages to the object cgroup without holding a
>   reference to the original memory cgroup.
> 
> ## Challenges
> 
> In a non-MGLRU scenario, each lruvec of every memory cgroup comprises four
> LRU lists (i.e., two active lists for anonymous and file folios, and two
> inactive lists for anonymous and file folios). Due to the symmetry of the
> LRU lists, it is feasible to transfer the LRU lists from a memory cgroup
> to its parent memory cgroup during the reparenting process.
> 
> In a MGLRU scenario, each lruvec of every memory cgroup comprises at least
> 2 (MIN_NR_GENS) generations and at most 4 (MAX_NR_GENS) generations.
> 
> 1. The first question is how to move the LRU lists from a memory cgroup to
>   its parent memory cgroup during the reparenting process. This is due to
>   the fact that the quantity of LRU lists (aka generations) may differ
>   between a child memory cgroup and its parent memory cgroup.
> 
> 2. The second question is how to make the process of reparenting more
>   efficient, since each folio charged to a memory cgroup stores its
>   generation counter into its ->flags. And the generation counter may
>   differ between a child memory cgroup and its parent memory cgroup because
>   the values of ->min_seq and ->max_seq are not identical. Should those
>   generation counters be updated correspondingly?
> 
> I am uncertain about how to handle them appropriately as I am not an
> expert at MGLRU. I would appreciate it if you could offer some suggestions.
> Moreover, if you are willing to directly provide your patches, I would be
> glad to incorporate them into this patchset.
> 
> ## Compositions
> 
> Patches 1-8 involve code refactoring and cleanup with the aim of
> facilitating the transfer LRU folios to object cgroup infrastructures.
> 
> Patches 9-10 aim to allocate the object cgroup for non-kmem scenarios,
> enabling the ability that LRU folios could be charged to it and aligning
> the behavior of object-cgroup-related APIs with that of the memory cgroup.
> 
> Patches 11-19 aim to prevent memory cgroup returned by folio_memcg() from
> being released.
> 
> Patches 20-23 aim to prevent lruvec returned by folio_lruvec() from being
> released.
> 
> Patches 24-25 implement the core mechanism to guarantee binding stability
> between the folio and its corresponding memory cgroup while holding lruvec
> lock or split-queue lock of THP.
> 
> Patches 26-27 are intended to transfer the LRU pages to the object cgroup
> without holding a reference to the original memory cgroup in order to
> address the issue of the dying memory cgroup.
> 
> Patch 28 aims to add VM_WARN_ON_ONCE_FOLIO to LRU maintenance helpers to
> ensure correct folio operations in the future.
> 
> ## Effect
> 
> Finally, it can be observed that the quantity of dying memory cgroups will
> not experience a significant increase if the following test script is
> executed to reproduce the issue.
> 
> ```bash
> #!/bin/bash
> 
> # Create a temporary file 'temp' filled with zero bytes
> dd if=/dev/zero of=temp bs=4096 count=1
> 
> # Display memory-cgroup info from /proc/cgroups
> cat /proc/cgroups | grep memory
> 
> for i in {0..2000}
> do
>    mkdir /sys/fs/cgroup/memory/test$i
>    echo $$ > /sys/fs/cgroup/memory/test$i/cgroup.procs
> 
>    # Append 'temp' file content to 'log'
>    cat temp >> log
> 
>    echo $$ > /sys/fs/cgroup/memory/cgroup.procs
> 
>    # Potentially create a dying memory cgroup
>    rmdir /sys/fs/cgroup/memory/test$i
> done
> 
> # Display memory-cgroup info after test
> cat /proc/cgroups | grep memory
> 
> rm -f temp log
> ```
> 
> ## References
> 
> [1] https://lore.kernel.org/linux-mm/Z6OkXXYDorPrBvEQ@hm-sls2/
> [2] https://lwn.net/Articles/895431/
> [3] https://github.com/systemd/systemd/pull/36827
> 
> Muchun Song (28):
>  mm: memcontrol: remove dead code of checking parent memory cgroup
>  mm: memcontrol: use folio_memcg_charged() to avoid potential rcu lock
>    holding
>  mm: workingset: use folio_lruvec() in workingset_refault()
>  mm: rename unlock_page_lruvec_irq and its variants
>  mm: thp: replace folio_memcg() with folio_memcg_charged()
>  mm: thp: introduce folio_split_queue_lock and its variants
>  mm: thp: use folio_batch to handle THP splitting in
>    deferred_split_scan()
>  mm: vmscan: refactor move_folios_to_lru()
>  mm: memcontrol: allocate object cgroup for non-kmem case
>  mm: memcontrol: return root object cgroup for root memory cgroup
>  mm: memcontrol: prevent memory cgroup release in
>    get_mem_cgroup_from_folio()
>  buffer: prevent memory cgroup release in folio_alloc_buffers()
>  writeback: prevent memory cgroup release in writeback module
>  mm: memcontrol: prevent memory cgroup release in
>    count_memcg_folio_events()
>  mm: page_io: prevent memory cgroup release in page_io module
>  mm: migrate: prevent memory cgroup release in folio_migrate_mapping()
>  mm: mglru: prevent memory cgroup release in mglru
>  mm: memcontrol: prevent memory cgroup release in
>    mem_cgroup_swap_full()
>  mm: workingset: prevent memory cgroup release in lru_gen_eviction()
>  mm: workingset: prevent lruvec release in workingset_refault()
>  mm: zswap: prevent lruvec release in zswap_folio_swapin()
>  mm: swap: prevent lruvec release in swap module
>  mm: workingset: prevent lruvec release in workingset_activation()
>  mm: memcontrol: prepare for reparenting LRU pages for lruvec lock
>  mm: thp: prepare for reparenting LRU pages for split queue lock
>  mm: memcontrol: introduce memcg_reparent_ops
>  mm: memcontrol: eliminate the problem of dying memory cgroup for LRU
>    folios
>  mm: lru: add VM_WARN_ON_ONCE_FOLIO to lru maintenance helpers
> 
> fs/buffer.c                      |   4 +-
> fs/fs-writeback.c                |  22 +-
> include/linux/memcontrol.h       | 190 ++++++------
> include/linux/mm_inline.h        |   6 +
> include/trace/events/writeback.h |   3 +
> mm/compaction.c                  |  43 ++-
> mm/huge_memory.c                 | 218 +++++++++-----
> mm/memcontrol-v1.c               |  15 +-
> mm/memcontrol.c                  | 476 +++++++++++++++++++------------
> mm/migrate.c                     |   2 +
> mm/mlock.c                       |   2 +-
> mm/page_io.c                     |   8 +-
> mm/percpu.c                      |   2 +-
> mm/shrinker.c                    |   6 +-
> mm/swap.c                        |  22 +-
> mm/vmscan.c                      |  73 ++---
> mm/workingset.c                  |  26 +-
> mm/zswap.c                       |   2 +
> 18 files changed, 696 insertions(+), 424 deletions(-)
> 
> -- 
> 2.20.1
>