mm, memcontrol: move swap charge handling into get_swap_page()

Patch series "mm, memcontrol: Implement memory.swap.events", v2.

This patchset implements memory.swap.events which contains max and fail
events so that userland can monitor and respond to swap running out.

This patch (of 2):

get_swap_page() is always followed by mem_cgroup_try_charge_swap().
This patch moves mem_cgroup_try_charge_swap() into get_swap_page() and
makes get_swap_page() call the function even after swap allocation
failure.

This simplifies the callers and consolidates memcg related logic and
will ease adding swap related memcg events.

Link: http://lkml.kernel.org/r/20180416230934.GH1911913@devbig577.frc2.facebook.com
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Roman Gushchin <guro@fb.com>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -6012,6 +6012,9 @@ int mem_cgroup_try_charge_swap(struct page *page, swp_entry_t entry)
 	if (!memcg)
 		return 0;
 
+	if (!entry.val)
+		return 0;
+
 	memcg = mem_cgroup_id_get_online(memcg);
 
 	if (!mem_cgroup_is_root(memcg) &&

mm/shmem.c

@@ -1322,9 +1322,6 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
 	if (!swap.val)
 		goto redirty;
 
-	if (mem_cgroup_try_charge_swap(page, swap))
-		goto free_swap;
-
 	/*
 	 * Add inode to shmem_unuse()'s list of swapped-out inodes,
 	 * if it's not already there.  Do it now before the page is
@@ -1353,7 +1350,6 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
 	}
 
 	mutex_unlock(&shmem_swaplist_mutex);
-free_swap:
 	put_swap_page(page, swap);
 redirty:
 	set_page_dirty(page);

mm/swap_slots.c

@@ -317,7 +317,7 @@ swp_entry_t get_swap_page(struct page *page)
 	if (PageTransHuge(page)) {
 		if (IS_ENABLED(CONFIG_THP_SWAP))
 			get_swap_pages(1, true, &entry);
-		return entry;
+		goto out;
 	}
 
 	/*
@@ -347,10 +347,14 @@ swp_entry_t get_swap_page(struct page *page)
 		}
 		mutex_unlock(&cache->alloc_lock);
 		if (entry.val)
-			return entry;
+			goto out;
 	}
 
 	get_swap_pages(1, false, &entry);
-
+out:
+	if (mem_cgroup_try_charge_swap(page, entry)) {
+		put_swap_page(page, entry);
+		entry.val = 0;
+	}
 	return entry;
 }

mm/swap_state.c

@@ -216,9 +216,6 @@ int add_to_swap(struct page *page)
 	if (!entry.val)
 		return 0;
 
-	if (mem_cgroup_try_charge_swap(page, entry))
-		goto fail;
-
 	/*
 	 * Radix-tree node allocations from PF_MEMALLOC contexts could
 	 * completely exhaust the page allocator. __GFP_NOMEMALLOC