On Fri, Mar 05, 2021 at 01:26:39PM -0800, Shakeel Butt wrote:
> Currently the kernel adds the page, allocated for swapin, to the
> swapcache before charging the page. This is fine but now we want a
> per-memcg swapcache stat which is essential for folks who wants to
> transparently migrate from cgroup v1's memsw to cgroup v2's memory and
> swap counters. In addition charging a page before exposing it to other
> parts of the kernel is a step in the right direction.
>
> To correctly maintain the per-memcg swapcache stat, this patch has
> adopted to charge the page before adding it to swapcache. One
> challenge in this option is the failure case of add_to_swap_cache() on
> which we need to undo the mem_cgroup_charge(). Specifically undoing
> mem_cgroup_uncharge_swap() is not simple.
>
> To resolve the issue, this patch introduces transaction like interface
> to charge a page for swapin. The function mem_cgroup_charge_swapin_page()
> initiates the charging of the page and mem_cgroup_finish_swapin_page()
> completes the charging process. So, the kernel starts the charging
> process of the page for swapin with mem_cgroup_charge_swapin_page(),
> adds the page to the swapcache and on success completes the charging
> process with mem_cgroup_finish_swapin_page().
>
> Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
Acked-by: Roman Gushchin <guro@xxxxxx>
Thanks!
> ---
> Changes since v3:
> - Updated the comments on introduced functions (Johannes)
> - Rename the funcations to be more clear (Hugh & Johannes)
>
> Changes since v2:
> - fixed build for !CONFIG_MEMCG
> - simplified failure path from add_to_swap_cache()
>
> Changes since v1:
> - Removes __GFP_NOFAIL and introduced transaction interface for charging
> (suggested by Johannes)
> - Updated the commit message
>
> include/linux/memcontrol.h | 13 +++++
> mm/memcontrol.c | 117 +++++++++++++++++++++++--------------
> mm/memory.c | 14 ++---
> mm/swap_state.c | 13 ++---
> 4 files changed, 97 insertions(+), 60 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index e6dc793d587d..f522b09f2df7 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -596,6 +596,9 @@ static inline bool mem_cgroup_below_min(struct mem_cgroup *memcg)
> }
>
> int mem_cgroup_charge(struct page *page, struct mm_struct *mm, gfp_t gfp_mask);
> +int mem_cgroup_swapin_charge_page(struct page *page, struct mm_struct *mm,
> + gfp_t gfp, swp_entry_t entry);
> +void mem_cgroup_swapin_uncharge_swap(swp_entry_t entry);
>
> void mem_cgroup_uncharge(struct page *page);
> void mem_cgroup_uncharge_list(struct list_head *page_list);
> @@ -1141,6 +1144,16 @@ static inline int mem_cgroup_charge(struct page *page, struct mm_struct *mm,
> return 0;
> }
>
> +static inline int mem_cgroup_swapin_charge_page(struct page *page,
> + struct mm_struct *mm, gfp_t gfp, swp_entry_t entry)
> +{
> + return 0;
> +}
> +
> +static inline void mem_cgroup_swapin_uncharge_swap(swp_entry_t entry)
> +{
> +}
> +
> static inline void mem_cgroup_uncharge(struct page *page)
> {
> }
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 2db2aeac8a9e..21c38c0b6e5a 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6690,6 +6690,27 @@ void mem_cgroup_calculate_protection(struct mem_cgroup *root,
> atomic_long_read(&parent->memory.children_low_usage)));
> }
>
> +static int __mem_cgroup_charge(struct page *page, struct mem_cgroup *memcg,
> + gfp_t gfp)
> +{
> + unsigned int nr_pages = thp_nr_pages(page);
> + int ret;
> +
> + ret = try_charge(memcg, gfp, nr_pages);
> + if (ret)
> + goto out;
> +
> + css_get(&memcg->css);
> + commit_charge(page, memcg);
> +
> + local_irq_disable();
> + mem_cgroup_charge_statistics(memcg, page, nr_pages);
> + memcg_check_events(memcg, page);
> + local_irq_enable();
> +out:
> + return ret;
> +}
> +
> /**
> * mem_cgroup_charge - charge a newly allocated page to a cgroup
> * @page: page to charge
> @@ -6699,55 +6720,71 @@ void mem_cgroup_calculate_protection(struct mem_cgroup *root,
> * Try to charge @page to the memcg that @mm belongs to, reclaiming
> * pages according to @gfp_mask if necessary.
> *
> + * Do not use this for pages allocated for swapin.
> + *
> * Returns 0 on success. Otherwise, an error code is returned.
> */
> int mem_cgroup_charge(struct page *page, struct mm_struct *mm, gfp_t gfp_mask)
> {
> - unsigned int nr_pages = thp_nr_pages(page);
> - struct mem_cgroup *memcg = NULL;
> - int ret = 0;
> + struct mem_cgroup *memcg;
> + int ret;
>
> if (mem_cgroup_disabled())
> - goto out;
> + return 0;
>
> - if (PageSwapCache(page)) {
> - swp_entry_t ent = { .val = page_private(page), };
> - unsigned short id;
> + memcg = get_mem_cgroup_from_mm(mm);
> + ret = __mem_cgroup_charge(page, memcg, gfp_mask);
> + css_put(&memcg->css);
>
> - /*
> - * Every swap fault against a single page tries to charge the
> - * page, bail as early as possible. shmem_unuse() encounters
> - * already charged pages, too. page and memcg binding is
> - * protected by the page lock, which serializes swap cache
> - * removal, which in turn serializes uncharging.
> - */
> - VM_BUG_ON_PAGE(!PageLocked(page), page);
> - if (page_memcg(compound_head(page)))
> - goto out;
> + return ret;
> +}
>
> - id = lookup_swap_cgroup_id(ent);
> - rcu_read_lock();
> - memcg = mem_cgroup_from_id(id);
> - if (memcg && !css_tryget_online(&memcg->css))
> - memcg = NULL;
> - rcu_read_unlock();
> - }
> +/**
> + * mem_cgroup_swapin_charge_page - charge a newly allocated page for swapin
> + * @page: page to charge
> + * @mm: mm context of the victim
> + * @gfp: reclaim mode
> + * @entry: swap entry for which the page is allocated
> + *
> + * This function charges a page allocated for swapin. Please call this before
> + * adding the page to the swapcache.
> + *
> + * Returns 0 on success. Otherwise, an error code is returned.
> + */
> +int mem_cgroup_swapin_charge_page(struct page *page, struct mm_struct *mm,
> + gfp_t gfp, swp_entry_t entry)
> +{
> + struct mem_cgroup *memcg;
> + unsigned short id;
> + int ret;
>
> - if (!memcg)
> + if (mem_cgroup_disabled())
> + return 0;
> +
> + id = lookup_swap_cgroup_id(entry);
> + rcu_read_lock();
> + memcg = mem_cgroup_from_id(id);
> + if (!memcg || !css_tryget_online(&memcg->css))
> memcg = get_mem_cgroup_from_mm(mm);
> + rcu_read_unlock();
>
> - ret = try_charge(memcg, gfp_mask, nr_pages);
> - if (ret)
> - goto out_put;
> + ret = __mem_cgroup_charge(page, memcg, gfp);
>
> - css_get(&memcg->css);
> - commit_charge(page, memcg);
> -
> - local_irq_disable();
> - mem_cgroup_charge_statistics(memcg, page, nr_pages);
> - memcg_check_events(memcg, page);
> - local_irq_enable();
> + css_put(&memcg->css);
> + return ret;
> +}
>
> +/*
> + * mem_cgroup_swapin_uncharge_swap - uncharge swap slot
> + * @entry: swap entry for which the page is charged
> + *
> + * Call this function after successfully adding the charged page to swapcache.
> + *
> + * Note: This function assumes the page for which swap slot is being uncharged
> + * is order 0 page.
> + */
> +void mem_cgroup_swapin_uncharge_swap(swp_entry_t entry)
> +{
> /*
> * Cgroup1's unified memory+swap counter has been charged with the
> * new swapcache page, finish the transfer by uncharging the swap
> @@ -6760,20 +6797,14 @@ int mem_cgroup_charge(struct page *page, struct mm_struct *mm, gfp_t gfp_mask)
> * correspond 1:1 to page and swap slot lifetimes: we charge the
> * page to memory here, and uncharge swap when the slot is freed.
> */
> - if (do_memsw_account() && PageSwapCache(page)) {
> - swp_entry_t entry = { .val = page_private(page) };
> + if (!mem_cgroup_disabled() && do_memsw_account()) {
> /*
> * The swap entry might not get freed for a long time,
> * let's not wait for it. The page already received a
> * memory+swap charge, drop the swap entry duplicate.
> */
> - mem_cgroup_uncharge_swap(entry, nr_pages);
> + mem_cgroup_uncharge_swap(entry, 1);
> }
> -
> -out_put:
> - css_put(&memcg->css);
> -out:
> - return ret;
> }
>
> struct uncharge_gather {
> diff --git a/mm/memory.c b/mm/memory.c
> index c8e357627318..5ddc133d0038 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3307,21 +3307,15 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma,
> vmf->address);
> if (page) {
> - int err;
> -
> __SetPageLocked(page);
> __SetPageSwapBacked(page);
> - set_page_private(page, entry.val);
> -
> - /* Tell memcg to use swap ownership records */
> - SetPageSwapCache(page);
> - err = mem_cgroup_charge(page, vma->vm_mm,
> - GFP_KERNEL);
> - ClearPageSwapCache(page);
> - if (err) {
> +
> + if (mem_cgroup_swapin_charge_page(page,
> + vma->vm_mm, GFP_KERNEL, entry)) {
> ret = VM_FAULT_OOM;
> goto out_page;
> }
> + mem_cgroup_swapin_uncharge_swap(entry);
>
> shadow = get_shadow_from_swap_cache(entry);
> if (shadow)
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 3cdee7b11da9..fb7efa08fe57 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -497,16 +497,14 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> __SetPageLocked(page);
> __SetPageSwapBacked(page);
>
> - /* May fail (-ENOMEM) if XArray node allocation failed. */
> - if (add_to_swap_cache(page, entry, gfp_mask & GFP_RECLAIM_MASK, &shadow)) {
> - put_swap_page(page, entry);
> + if (mem_cgroup_swapin_charge_page(page, NULL, gfp_mask, entry))
> goto fail_unlock;
> - }
>
> - if (mem_cgroup_charge(page, NULL, gfp_mask)) {
> - delete_from_swap_cache(page);
> + /* May fail (-ENOMEM) if XArray node allocation failed. */
> + if (add_to_swap_cache(page, entry, gfp_mask & GFP_RECLAIM_MASK, &shadow))
> goto fail_unlock;
> - }
> +
> + mem_cgroup_swapin_uncharge_swap(entry);
>
> if (shadow)
> workingset_refault(page, shadow);
> @@ -517,6 +515,7 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> return page;
>
> fail_unlock:
> + put_swap_page(page, entry);
> unlock_page(page);
> put_page(page);
> return NULL;
> --
> 2.30.1.766.gb4fecdf3b7-goog
>
