On Tue, May 24, 2022 at 02:05:47PM +0800, Muchun Song wrote:
> When we use objcg APIs to charge the LRU pages, the page will not hold
> a reference to the memcg associated with the page. So the caller of the
> {folio,page}_memcg() should hold an rcu read lock or obtain a reference
> to the memcg associated with the page to protect memcg from being
> released. So introduce get_mem_cgroup_from_{page,folio}() to obtain a
> reference to the memory cgroup associated with the page.
>
> In this patch, make all the callers hold an rcu read lock or obtain a
> reference to the memcg to protect memcg from being released when the LRU
> pages reparented.
>
> We do not need to adjust the callers of {folio,page}_memcg() during
> the whole process of mem_cgroup_move_task(). Because the cgroup migration
> and memory cgroup offlining are serialized by @cgroup_mutex. In this
> routine, the LRU pages cannot be reparented to its parent memory cgroup.
> So {folio,page}_memcg() is stable and cannot be released.
>
> This is a preparation for reparenting the LRU pages.
>
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Overall the patch looks good to me (some nits below).
> ---
> fs/buffer.c | 4 +--
> fs/fs-writeback.c | 23 ++++++++--------
> include/linux/memcontrol.h | 51 ++++++++++++++++++++++++++++++++---
> include/trace/events/writeback.h | 5 ++++
> mm/memcontrol.c | 58 ++++++++++++++++++++++++++++++----------
> mm/migrate.c | 4 +++
> mm/page_io.c | 5 ++--
> 7 files changed, 117 insertions(+), 33 deletions(-)
>
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 2b5561ae5d0b..80975a457670 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -819,8 +819,7 @@ struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
> if (retry)
> gfp |= __GFP_NOFAIL;
>
> - /* The page lock pins the memcg */
> - memcg = page_memcg(page);
> + memcg = get_mem_cgroup_from_page(page);
Looking at these changes I wonder if we need to remove unsafe getters or
at least add a BOLD comment on how/when it's safe to use them.
> old_memcg = set_active_memcg(memcg);
>
> head = NULL;
> @@ -840,6 +839,7 @@ struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
> set_bh_page(bh, page, offset);
> }
> out:
> + mem_cgroup_put(memcg);
> set_active_memcg(old_memcg);
> return head;
> /*
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 1fae0196292a..56612ace8778 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -243,15 +243,13 @@ void __inode_attach_wb(struct inode *inode, struct page *page)
> if (inode_cgwb_enabled(inode)) {
> struct cgroup_subsys_state *memcg_css;
>
> - if (page) {
> - memcg_css = mem_cgroup_css_from_page(page);
> - wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
> - } else {
> - /* must pin memcg_css, see wb_get_create() */
> + /* must pin memcg_css, see wb_get_create() */
> + if (page)
> + memcg_css = get_mem_cgroup_css_from_page(page);
> + else
> memcg_css = task_get_css(current, memory_cgrp_id);
> - wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
> - css_put(memcg_css);
> - }
> + wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
> + css_put(memcg_css);
> }
>
> if (!wb)
> @@ -868,16 +866,16 @@ void wbc_account_cgroup_owner(struct writeback_control *wbc, struct page *page,
> if (!wbc->wb || wbc->no_cgroup_owner)
> return;
>
> - css = mem_cgroup_css_from_page(page);
> + css = get_mem_cgroup_css_from_page(page);
> /* dead cgroups shouldn't contribute to inode ownership arbitration */
> if (!(css->flags & CSS_ONLINE))
> - return;
> + goto out;
>
> id = css->id;
>
> if (id == wbc->wb_id) {
> wbc->wb_bytes += bytes;
> - return;
> + goto out;
> }
>
> if (id == wbc->wb_lcand_id)
> @@ -890,6 +888,9 @@ void wbc_account_cgroup_owner(struct writeback_control *wbc, struct page *page,
> wbc->wb_tcand_bytes += bytes;
> else
> wbc->wb_tcand_bytes -= min(bytes, wbc->wb_tcand_bytes);
> +
> +out:
> + css_put(css);
> }
> EXPORT_SYMBOL_GPL(wbc_account_cgroup_owner);
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 8c2f1ba2f471..3a0e2592434e 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -373,7 +373,7 @@ static inline bool folio_memcg_kmem(struct folio *folio);
> * a valid memcg, but can be atomically swapped to the parent memcg.
> *
> * The caller must ensure that the returned memcg won't be released:
> - * e.g. acquire the rcu_read_lock or css_set_lock.
> + * e.g. acquire the rcu_read_lock or objcg_lock or cgroup_mutex.
> */
> static inline struct mem_cgroup *obj_cgroup_memcg(struct obj_cgroup *objcg)
> {
> @@ -454,7 +454,37 @@ static inline struct mem_cgroup *page_memcg(struct page *page)
> return folio_memcg(page_folio(page));
> }
>
> -/**
> +/*
> + * get_mem_cgroup_from_folio - Obtain a reference on the memory cgroup
> + * associated with a folio.
> + * @folio: Pointer to the folio.
> + *
> + * Returns a pointer to the memory cgroup (and obtain a reference on it)
> + * associated with the folio, or NULL. This function assumes that the
> + * folio is known to have a proper memory cgroup pointer. It's not safe
> + * to call this function against some type of pages, e.g. slab pages or
> + * ex-slab pages.
> + */
> +static inline struct mem_cgroup *get_mem_cgroup_from_folio(struct folio *folio)
> +{
> + struct mem_cgroup *memcg;
> +
> + rcu_read_lock();
> +retry:
> + memcg = folio_memcg(folio);
> + if (unlikely(memcg && !css_tryget(&memcg->css)))
> + goto retry;
> + rcu_read_unlock();
> +
> + return memcg;
> +}
> +
> +static inline struct mem_cgroup *get_mem_cgroup_from_page(struct page *page)
> +{
> + return get_mem_cgroup_from_folio(page_folio(page));
> +}
> +
> +/*
> * folio_memcg_rcu - Locklessly get the memory cgroup associated with a folio.
> * @folio: Pointer to the folio.
> *
> @@ -873,7 +903,7 @@ static inline bool mm_match_cgroup(struct mm_struct *mm,
> return match;
> }
>
> -struct cgroup_subsys_state *mem_cgroup_css_from_page(struct page *page);
> +struct cgroup_subsys_state *get_mem_cgroup_css_from_page(struct page *page);
> ino_t page_cgroup_ino(struct page *page);
>
> static inline bool mem_cgroup_online(struct mem_cgroup *memcg)
> @@ -1047,10 +1077,13 @@ static inline void count_memcg_events(struct mem_cgroup *memcg,
> static inline void count_memcg_page_event(struct page *page,
> enum vm_event_item idx)
> {
> - struct mem_cgroup *memcg = page_memcg(page);
> + struct mem_cgroup *memcg;
>
> + rcu_read_lock();
> + memcg = page_memcg(page);
> if (memcg)
> count_memcg_events(memcg, idx, 1);
> + rcu_read_unlock();
> }
>
> static inline void count_memcg_event_mm(struct mm_struct *mm,
> @@ -1129,6 +1162,16 @@ static inline struct mem_cgroup *page_memcg(struct page *page)
> return NULL;
> }
>
> +static inline struct mem_cgroup *get_mem_cgroup_from_folio(struct folio *folio)
> +{
> + return NULL;
> +}
> +
> +static inline struct mem_cgroup *get_mem_cgroup_from_page(struct page *page)
> +{
> + return NULL;
> +}
> +
> static inline struct mem_cgroup *folio_memcg_rcu(struct folio *folio)
> {
> WARN_ON_ONCE(!rcu_read_lock_held());
> diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
> index 86b2a82da546..cdb822339f13 100644
> --- a/include/trace/events/writeback.h
> +++ b/include/trace/events/writeback.h
> @@ -258,6 +258,11 @@ TRACE_EVENT(track_foreign_dirty,
> __entry->ino = inode ? inode->i_ino : 0;
> __entry->memcg_id = wb->memcg_css->id;
> __entry->cgroup_ino = __trace_wb_assign_cgroup(wb);
> + /*
> + * TP_fast_assign() is under preemption disabled which can
> + * serve as an RCU read-side critical section so that the
> + * memcg returned by folio_memcg() cannot be freed.
> + */
> __entry->page_cgroup_ino = cgroup_ino(folio_memcg(folio)->css.cgroup);
> ),
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index b38a77f6696f..dcaf6cf5dc74 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -371,7 +371,7 @@ EXPORT_SYMBOL(memcg_kmem_enabled_key);
> #endif
>
> /**
> - * mem_cgroup_css_from_page - css of the memcg associated with a page
> + * get_mem_cgroup_css_from_page - get css of the memcg associated with a page
> * @page: page of interest
> *
> * If memcg is bound to the default hierarchy, css of the memcg associated
> @@ -381,13 +381,15 @@ EXPORT_SYMBOL(memcg_kmem_enabled_key);
> * If memcg is bound to a traditional hierarchy, the css of root_mem_cgroup
> * is returned.
> */
> -struct cgroup_subsys_state *mem_cgroup_css_from_page(struct page *page)
> +struct cgroup_subsys_state *get_mem_cgroup_css_from_page(struct page *page)
> {
> struct mem_cgroup *memcg;
>
> - memcg = page_memcg(page);
> + if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
> + return &root_mem_cgroup->css;
>
> - if (!memcg || !cgroup_subsys_on_dfl(memory_cgrp_subsys))
> + memcg = get_mem_cgroup_from_page(page);
> + if (!memcg)
> memcg = root_mem_cgroup;
>
> return &memcg->css;
> @@ -770,13 +772,13 @@ void __mod_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
> void __mod_lruvec_page_state(struct page *page, enum node_stat_item idx,
> int val)
> {
> - struct page *head = compound_head(page); /* rmap on tail pages */
> + struct folio *folio = page_folio(page); /* rmap on tail pages */
> struct mem_cgroup *memcg;
> pg_data_t *pgdat = page_pgdat(page);
> struct lruvec *lruvec;
>
> rcu_read_lock();
> - memcg = page_memcg(head);
> + memcg = folio_memcg(folio);
> /* Untracked pages have no memcg, no lruvec. Update only the node */
> if (!memcg) {
> rcu_read_unlock();
> @@ -2058,7 +2060,9 @@ void folio_memcg_lock(struct folio *folio)
> * The RCU lock is held throughout the transaction. The fast
> * path can get away without acquiring the memcg->move_lock
> * because page moving starts with an RCU grace period.
> - */
> + *
> + * The RCU lock also protects the memcg from being freed.
> + */
> rcu_read_lock();
>
> if (mem_cgroup_disabled())
> @@ -3296,7 +3300,7 @@ void obj_cgroup_uncharge(struct obj_cgroup *objcg, size_t size)
> void split_page_memcg(struct page *head, unsigned int nr)
> {
> struct folio *folio = page_folio(head);
> - struct mem_cgroup *memcg = folio_memcg(folio);
> + struct mem_cgroup *memcg = get_mem_cgroup_from_folio(folio);
> int i;
>
> if (mem_cgroup_disabled() || !memcg)
> @@ -3309,6 +3313,8 @@ void split_page_memcg(struct page *head, unsigned int nr)
> obj_cgroup_get_many(__folio_objcg(folio), nr - 1);
> else
> css_get_many(&memcg->css, nr - 1);
> +
> + css_put(&memcg->css);
> }
>
> #ifdef CONFIG_MEMCG_SWAP
> @@ -4511,7 +4517,7 @@ void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
> void mem_cgroup_track_foreign_dirty_slowpath(struct folio *folio,
> struct bdi_writeback *wb)
> {
> - struct mem_cgroup *memcg = folio_memcg(folio);
> + struct mem_cgroup *memcg = get_mem_cgroup_from_folio(folio);
> struct memcg_cgwb_frn *frn;
> u64 now = get_jiffies_64();
> u64 oldest_at = now;
> @@ -4558,6 +4564,7 @@ void mem_cgroup_track_foreign_dirty_slowpath(struct folio *folio,
> frn->memcg_id = wb->memcg_css->id;
> frn->at = now;
> }
> + css_put(&memcg->css);
> }
>
> /* issue foreign writeback flushes for recorded foreign dirtying events */
> @@ -6092,6 +6099,14 @@ static void mem_cgroup_move_charge(void)
> atomic_dec(&mc.from->moving_account);
> }
>
> +/*
> + * The cgroup migration and memory cgroup offlining are serialized by
> + * @cgroup_mutex. If we reach here, it means that the LRU pages cannot
> + * be reparented to its parent memory cgroup. So during the whole process
> + * of mem_cgroup_move_task(), page_memcg(page) is stable. So we do not
> + * need to worry about the memcg (returned from page_memcg()) being
> + * released even if we do not hold an rcu read lock.
> + */
> static void mem_cgroup_move_task(void)
> {
> if (mc.to) {
> @@ -6895,7 +6910,7 @@ void mem_cgroup_migrate(struct folio *old, struct folio *new)
> if (folio_memcg(new))
> return;
>
> - memcg = folio_memcg(old);
> + memcg = get_mem_cgroup_from_folio(old);
> VM_WARN_ON_ONCE_FOLIO(!memcg, old);
> if (!memcg)
> return;
> @@ -6914,6 +6929,8 @@ void mem_cgroup_migrate(struct folio *old, struct folio *new)
> mem_cgroup_charge_statistics(memcg, nr_pages);
> memcg_check_events(memcg, folio_nid(new));
> local_irq_restore(flags);
> +
> + css_put(&memcg->css);
> }
>
> DEFINE_STATIC_KEY_FALSE(memcg_sockets_enabled_key);
> @@ -7100,6 +7117,10 @@ void mem_cgroup_swapout(struct folio *folio, swp_entry_t entry)
> if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
> return;
>
> + /*
> + * Interrupts should be disabled by the caller (see the comments below),
> + * which can serve as RCU read-side critical sections.
> + */
> memcg = folio_memcg(folio);
>
> VM_WARN_ON_ONCE_FOLIO(!memcg, folio);
> @@ -7165,15 +7186,16 @@ int __mem_cgroup_try_charge_swap(struct page *page, swp_entry_t entry)
> if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
> return 0;
>
> + rcu_read_lock();
> memcg = page_memcg(page);
>
> VM_WARN_ON_ONCE_PAGE(!memcg, page);
> if (!memcg)
> - return 0;
> + goto out;
>
> if (!entry.val) {
> memcg_memory_event(memcg, MEMCG_SWAP_FAIL);
> - return 0;
> + goto out;
> }
>
> memcg = mem_cgroup_id_get_online(memcg);
> @@ -7183,6 +7205,7 @@ int __mem_cgroup_try_charge_swap(struct page *page, swp_entry_t entry)
> memcg_memory_event(memcg, MEMCG_SWAP_MAX);
> memcg_memory_event(memcg, MEMCG_SWAP_FAIL);
> mem_cgroup_id_put(memcg);
> + rcu_read_unlock();
> return -ENOMEM;
If you add the "out" label, please use it here too.
> }
>
> @@ -7192,6 +7215,8 @@ int __mem_cgroup_try_charge_swap(struct page *page, swp_entry_t entry)
> oldid = swap_cgroup_record(entry, mem_cgroup_id(memcg), nr_pages);
> VM_BUG_ON_PAGE(oldid, page);
> mod_memcg_state(memcg, MEMCG_SWAP, nr_pages);
> +out:
> + rcu_read_unlock();
>
> return 0;
> }
> @@ -7246,17 +7271,22 @@ bool mem_cgroup_swap_full(struct page *page)
> if (cgroup_memory_noswap || !cgroup_subsys_on_dfl(memory_cgrp_subsys))
> return false;
>
> + rcu_read_lock();
> memcg = page_memcg(page);
> if (!memcg)
> - return false;
> + goto out;
>
> for (; memcg != root_mem_cgroup; memcg = parent_mem_cgroup(memcg)) {
> unsigned long usage = page_counter_read(&memcg->swap);
>
> if (usage * 2 >= READ_ONCE(memcg->swap.high) ||
> - usage * 2 >= READ_ONCE(memcg->swap.max))
> + usage * 2 >= READ_ONCE(memcg->swap.max)) {
> + rcu_read_unlock();
> return true;
Please, make something like
ret = true;
goto out;
here. It will be more consistent.
> + }
> }
> +out:
> + rcu_read_unlock();
>
> return false;
> }
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 6c31ee1e1c9b..59e97a8a64a0 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -430,6 +430,10 @@ int folio_migrate_mapping(struct address_space *mapping,
> struct lruvec *old_lruvec, *new_lruvec;
> struct mem_cgroup *memcg;
>
> + /*
> + * Irq is disabled, which can serve as RCU read-side critical
> + * sections.
> + */
> memcg = folio_memcg(folio);
> old_lruvec = mem_cgroup_lruvec(memcg, oldzone->zone_pgdat);
> new_lruvec = mem_cgroup_lruvec(memcg, newzone->zone_pgdat);
> diff --git a/mm/page_io.c b/mm/page_io.c
> index 89fbf3cae30f..a0d9cd68e87a 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -221,13 +221,14 @@ static void bio_associate_blkg_from_page(struct bio *bio, struct page *page)
> struct cgroup_subsys_state *css;
> struct mem_cgroup *memcg;
>
> + rcu_read_lock();
> memcg = page_memcg(page);
> if (!memcg)
> - return;
> + goto out;
>
> - rcu_read_lock();
> css = cgroup_e_css(memcg->css.cgroup, &io_cgrp_subsys);
> bio_associate_blkg_from_css(bio, css);
> +out:
> rcu_read_unlock();
> }
> #else
> --
> 2.11.0
>
