Re: [PATCH v2 6/7] mm: free swap cache aggressively if memcg swap is full

[Michal Hocko <mhocko@kernel.org>]

On Thu 17-12-15 15:29:59, Vladimir Davydov wrote:
> Swap cache pages are freed aggressively if swap is nearly full (>50%
> currently), because otherwise we are likely to stop scanning anonymous
> when we near the swap limit even if there is plenty of freeable swap
> cache pages. We should follow the same trend in case of memory cgroup,
> which has its own swap limit.
> 
> Signed-off-by: Vladimir Davydov <vdavydov@virtuozzo.com>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>

I cannot say I would be deeply familiar with swapcache internalls and
all the interaction with the reclaim but from what I understand the
patch looks sane
Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> Changes in v2:
>  - Remove unnecessary PageSwapCache check from mem_cgroup_swap_full.
>  - Do not check swap limit on the legacy hierarchy.
> 
>  include/linux/swap.h |  6 ++++++
>  mm/memcontrol.c      | 22 ++++++++++++++++++++++
>  mm/memory.c          |  3 ++-
>  mm/swapfile.c        |  2 +-
>  mm/vmscan.c          |  2 +-
>  5 files changed, 32 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index c544998dfbe7..5ebdbabc62f0 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -552,6 +552,7 @@ extern void mem_cgroup_swapout(struct page *page, swp_entry_t entry);
>  extern int mem_cgroup_try_charge_swap(struct page *page, swp_entry_t entry);
>  extern void mem_cgroup_uncharge_swap(swp_entry_t entry);
>  extern long mem_cgroup_get_nr_swap_pages(struct mem_cgroup *memcg);
> +extern bool mem_cgroup_swap_full(struct page *page);
>  #else
>  static inline void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
>  {
> @@ -571,6 +572,11 @@ static inline long mem_cgroup_get_nr_swap_pages(struct mem_cgroup *memcg)
>  {
>  	return get_nr_swap_pages();
>  }
> +
> +static inline bool mem_cgroup_swap_full(struct page *page)
> +{
> +	return vm_swap_full();
> +}
>  #endif
>  
>  #endif /* __KERNEL__*/
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index e0e498f5ca32..fc25dc211eaf 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5749,6 +5749,28 @@ long mem_cgroup_get_nr_swap_pages(struct mem_cgroup *memcg)
>  	return nr_swap_pages;
>  }
>  
> +bool mem_cgroup_swap_full(struct page *page)
> +{
> +	struct mem_cgroup *memcg;
> +
> +	VM_BUG_ON_PAGE(!PageLocked(page), page);
> +
> +	if (vm_swap_full())
> +		return true;
> +	if (!do_swap_account || !cgroup_subsys_on_dfl(memory_cgrp_subsys))
> +		return false;
> +
> +	memcg = page->mem_cgroup;
> +	if (!memcg)
> +		return false;
> +
> +	for (; memcg != root_mem_cgroup; memcg = parent_mem_cgroup(memcg))
> +		if (page_counter_read(&memcg->swap) * 2 >= memcg->swap.limit)
> +			return true;
> +
> +	return false;
> +}
> +
>  /* for remember boot option*/
>  #ifdef CONFIG_MEMCG_SWAP_ENABLED
>  static int really_do_swap_account __initdata = 1;
> diff --git a/mm/memory.c b/mm/memory.c
> index 3b115dcaa26e..2bd6a78c142b 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2563,7 +2563,8 @@ int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  	}
>  
>  	swap_free(entry);
> -	if (vm_swap_full() || (vma->vm_flags & VM_LOCKED) || PageMlocked(page))
> +	if (mem_cgroup_swap_full(page) ||
> +	    (vma->vm_flags & VM_LOCKED) || PageMlocked(page))
>  		try_to_free_swap(page);
>  	unlock_page(page);
>  	if (page != swapcache) {
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index efa279221302..ab1a8a619676 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1009,7 +1009,7 @@ int free_swap_and_cache(swp_entry_t entry)
>  		 * Also recheck PageSwapCache now page is locked (above).
>  		 */
>  		if (PageSwapCache(page) && !PageWriteback(page) &&
> -				(!page_mapped(page) || vm_swap_full())) {
> +		    (!page_mapped(page) || mem_cgroup_swap_full(page))) {
>  			delete_from_swap_cache(page);
>  			SetPageDirty(page);
>  		}
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index ab52d865d922..1cd88e9b0383 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1206,7 +1206,7 @@ cull_mlocked:
>  
>  activate_locked:
>  		/* Not a candidate for swapping, so reclaim swap space. */
> -		if (PageSwapCache(page) && vm_swap_full())
> +		if (PageSwapCache(page) && mem_cgroup_swap_full(page))
>  			try_to_free_swap(page);
>  		VM_BUG_ON_PAGE(PageActive(page), page);
>  		SetPageActive(page);
> -- 
> 2.1.4

-- 
Michal Hocko
SUSE Labs