Re: [PATCH 05/18] mm: memcontrol: convert page cache to a new mem_cgroup_charge() API

[Joonsoo Kim]

On Mon, Apr 20, 2020 at 06:11:13PM -0400, Johannes Weiner wrote:
> The try/commit/cancel protocol that memcg uses dates back to when
> pages used to be uncharged upon removal from the page cache, and thus
> couldn't be committed before the insertion had succeeded. Nowadays,
> pages are uncharged when they are physically freed; it doesn't matter
> whether the insertion was successful or not. For the page cache, the
> transaction dance has become unnecessary.
> 
> Introduce a mem_cgroup_charge() function that simply charges a newly
> allocated page to a cgroup and sets up page->mem_cgroup in one single
> step. If the insertion fails, the caller doesn't have to do anything
> but free/put the page.
> 
> Then switch the page cache over to this new API.
> 
> Subsequent patches will also convert anon pages, but it needs a bit
> more prep work. Right now, memcg depends on page->mapping being
> already set up at the time of charging, so that it can maintain its
> own MEMCG_CACHE and MEMCG_RSS counters. For anon, page->mapping is set
> under the same pte lock under which the page is publishd, so a single
> charge point that can block doesn't work there just yet.
> 
> The following prep patches will replace the private memcg counters
> with the generic vmstat counters, thus removing the page->mapping
> dependency, then complete the transition to the new single-point
> charge API and delete the old transactional scheme.
> 
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> ---
>  include/linux/memcontrol.h | 10 ++++
>  mm/filemap.c               | 24 ++++------
>  mm/memcontrol.c            | 27 +++++++++++
>  mm/shmem.c                 | 97 +++++++++++++++++---------------------
>  4 files changed, 89 insertions(+), 69 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index c7875a48c8c1..5e8b0e38f145 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -367,6 +367,10 @@ int mem_cgroup_try_charge_delay(struct page *page, struct mm_struct *mm,
>  void mem_cgroup_commit_charge(struct page *page, struct mem_cgroup *memcg,
>  			      bool lrucare);
>  void mem_cgroup_cancel_charge(struct page *page, struct mem_cgroup *memcg);
> +
> +int mem_cgroup_charge(struct page *page, struct mm_struct *mm, gfp_t gfp_mask,
> +		      bool lrucare);
> +
>  void mem_cgroup_uncharge(struct page *page);
>  void mem_cgroup_uncharge_list(struct list_head *page_list);
>  
> @@ -872,6 +876,12 @@ static inline void mem_cgroup_cancel_charge(struct page *page,
>  {
>  }
>  
> +static inline int mem_cgroup_charge(struct page *page, struct mm_struct *mm,
> +				    gfp_t gfp_mask, bool lrucare)
> +{
> +	return 0;
> +}
> +
>  static inline void mem_cgroup_uncharge(struct page *page)
>  {
>  }
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 5b31af9d5b1b..5bdbda965177 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -832,7 +832,6 @@ static int __add_to_page_cache_locked(struct page *page,
>  {
>  	XA_STATE(xas, &mapping->i_pages, offset);
>  	int huge = PageHuge(page);
> -	struct mem_cgroup *memcg;
>  	int error;
>  	void *old;
>  
> @@ -840,17 +839,16 @@ static int __add_to_page_cache_locked(struct page *page,
>  	VM_BUG_ON_PAGE(PageSwapBacked(page), page);
>  	mapping_set_update(&xas, mapping);
>  
> -	if (!huge) {
> -		error = mem_cgroup_try_charge(page, current->mm,
> -					      gfp_mask, &memcg);
> -		if (error)
> -			return error;
> -	}
> -
>  	get_page(page);
>  	page->mapping = mapping;
>  	page->index = offset;
>  
> +	if (!huge) {
> +		error = mem_cgroup_charge(page, current->mm, gfp_mask, false);
> +		if (error)
> +			goto error;
> +	}
> +
>  	do {
>  		xas_lock_irq(&xas);
>  		old = xas_load(&xas);
> @@ -874,20 +872,18 @@ static int __add_to_page_cache_locked(struct page *page,
>  		xas_unlock_irq(&xas);
>  	} while (xas_nomem(&xas, gfp_mask & GFP_RECLAIM_MASK));
>  
> -	if (xas_error(&xas))
> +	if (xas_error(&xas)) {
> +		error = xas_error(&xas);
>  		goto error;
> +	}
>  
> -	if (!huge)
> -		mem_cgroup_commit_charge(page, memcg, false);
>  	trace_mm_filemap_add_to_page_cache(page);
>  	return 0;
>  error:
>  	page->mapping = NULL;
>  	/* Leave page->index set: truncation relies upon it */
> -	if (!huge)
> -		mem_cgroup_cancel_charge(page, memcg);
>  	put_page(page);
> -	return xas_error(&xas);
> +	return error;
>  }
>  ALLOW_ERROR_INJECTION(__add_to_page_cache_locked, ERRNO);
>  
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 711d6dd5cbb1..b38c0a672d26 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6577,6 +6577,33 @@ void mem_cgroup_cancel_charge(struct page *page, struct mem_cgroup *memcg)
>  	cancel_charge(memcg, nr_pages);
>  }
>  
> +/**
> + * mem_cgroup_charge - charge a newly allocated page to a cgroup
> + * @page: page to charge
> + * @mm: mm context of the victim
> + * @gfp_mask: reclaim mode
> + * @lrucare: page might be on the LRU already
> + *
> + * Try to charge @page to the memcg that @mm belongs to, reclaiming
> + * pages according to @gfp_mask if necessary.
> + *
> + * 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,
> +		      bool lrucare)
> +{
> +	struct mem_cgroup *memcg;
> +	int ret;
> +
> +	VM_BUG_ON_PAGE(!page->mapping, page);
> +
> +	ret = mem_cgroup_try_charge(page, mm, gfp_mask, &memcg);
> +	if (ret)
> +		return ret;
> +	mem_cgroup_commit_charge(page, memcg, lrucare);
> +	return 0;
> +}
> +
>  struct uncharge_gather {
>  	struct mem_cgroup *memcg;
>  	unsigned long pgpgout;
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 52c66801321e..2384f6c7ef71 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -605,11 +605,13 @@ static inline bool is_huge_enabled(struct shmem_sb_info *sbinfo)
>   */
>  static int shmem_add_to_page_cache(struct page *page,
>  				   struct address_space *mapping,
> -				   pgoff_t index, void *expected, gfp_t gfp)
> +				   pgoff_t index, void *expected, gfp_t gfp,
> +				   struct mm_struct *charge_mm)
>  {
>  	XA_STATE_ORDER(xas, &mapping->i_pages, index, compound_order(page));
>  	unsigned long i = 0;
>  	unsigned long nr = compound_nr(page);
> +	int error;
>  
>  	VM_BUG_ON_PAGE(PageTail(page), page);
>  	VM_BUG_ON_PAGE(index != round_down(index, nr), page);
> @@ -621,6 +623,16 @@ static int shmem_add_to_page_cache(struct page *page,
>  	page->mapping = mapping;
>  	page->index = index;
>  
> +	error = mem_cgroup_charge(page, charge_mm, gfp, PageSwapCache(page));
> +	if (error) {
> +		if (!PageSwapCache(page) && PageTransHuge(page)) {
> +			count_vm_event(THP_FILE_FALLBACK);
> +			count_vm_event(THP_FILE_FALLBACK_CHARGE);
> +		}
> +		goto error;
> +	}
> +	cgroup_throttle_swaprate(page, gfp);
> +
>  	do {
>  		void *entry;
>  		xas_lock_irq(&xas);
> @@ -648,12 +660,15 @@ static int shmem_add_to_page_cache(struct page *page,
>  	} while (xas_nomem(&xas, gfp));
>  
>  	if (xas_error(&xas)) {
> -		page->mapping = NULL;
> -		page_ref_sub(page, nr);
> -		return xas_error(&xas);
> +		error = xas_error(&xas);
> +		goto error;
>  	}
>  
>  	return 0;
> +error:
> +	page->mapping = NULL;
> +	page_ref_sub(page, nr);
> +	return error;
>  }
>  
>  /*
> @@ -1619,7 +1634,6 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
>  	struct address_space *mapping = inode->i_mapping;
>  	struct shmem_inode_info *info = SHMEM_I(inode);
>  	struct mm_struct *charge_mm = vma ? vma->vm_mm : current->mm;
> -	struct mem_cgroup *memcg;
>  	struct page *page;
>  	swp_entry_t swap;
>  	int error;
> @@ -1664,29 +1678,22 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
>  			goto failed;
>  	}
>  
> -	error = mem_cgroup_try_charge_delay(page, charge_mm, gfp, &memcg);
> -	if (!error) {
> -		error = shmem_add_to_page_cache(page, mapping, index,
> -						swp_to_radix_entry(swap), gfp);
> -		/*
> -		 * We already confirmed swap under page lock, and make
> -		 * no memory allocation here, so usually no possibility
> -		 * of error; but free_swap_and_cache() only trylocks a
> -		 * page, so it is just possible that the entry has been
> -		 * truncated or holepunched since swap was confirmed.
> -		 * shmem_undo_range() will have done some of the
> -		 * unaccounting, now delete_from_swap_cache() will do
> -		 * the rest.
> -		 */
> -		if (error) {
> -			mem_cgroup_cancel_charge(page, memcg);
> -			delete_from_swap_cache(page);
> -		}
> -	}
> -	if (error)
> +	error = shmem_add_to_page_cache(page, mapping, index,
> +					swp_to_radix_entry(swap), gfp,
> +					charge_mm);
> +	/*
> +	 * We already confirmed swap under page lock, and make no
> +	 * memory allocation here, so usually no possibility of error;
> +	 * but free_swap_and_cache() only trylocks a page, so it is
> +	 * just possible that the entry has been truncated or
> +	 * holepunched since swap was confirmed.  shmem_undo_range()
> +	 * will have done some of the unaccounting, now
> +	 * delete_from_swap_cache() will do the rest.
> +	 */
> +	if (error) {
> +		delete_from_swap_cache(page);
>  		goto failed;

-EEXIST (from swap cache) and -ENOMEM (from memcg) should be handled
differently. delete_from_swap_cache() is for -EEXIST case.

Thanks.