mm: use special value SHRINKER_REGISTERING instead of list_empty() check

The patch introduces a special value SHRINKER_REGISTERING to use instead
of list_empty() to differ a registering shrinker from unregistered
shrinker.  Why we need that at all?

Shrinker registration is split in two parts.  The first one is
prealloc_shrinker(), which allocates shrinker memory and reserves ID in
shrinker_idr.  This function can fail.  The second is
register_shrinker_prepared(), and it finalizes the registration.  This
function actually makes shrinker available to be used from
shrink_slab(), and it can't fail.

One shrinker may be based on more then one LRU lists.  So, we never
clear the bit in memcg shrinker maps, when (one of) corresponding LRU
list becomes empty, since other LRU lists may be not empty.  See
superblock shrinker for example: it is based on two LRU lists:
s_inode_lru and s_dentry_lru.  We do not want to clear shrinker bit,
when there are no inodes in s_inode_lru, as s_dentry_lru may contain
dentries.

Instead of that, we use special algorithm to detect shrinkers having no
elements at all its LRU lists, and this is made in shrink_slab_memcg().
See the comment in this function for the details.

Also, in shrink_slab_memcg() we clear shrinker bit in the map, when we
meet unregistered shrinker (bit is set, while there is no a shrinker in
IDR).  Otherwise, we would have done that at the moment of shrinker
unregistration for all memcgs (and this looks worse, since iteration
over all memcg may take much time).  Also this would have imposed
restrictions on shrinker unregistration order for its users: they would
have had to guarantee, there are no new elements after
unregister_shrinker() (otherwise, a new added element would have set a
bit).

So, if we meet a set bit in map and no shrinker in IDR when we're
iterating over the map in shrink_slab_memcg(), this means the
corresponding shrinker is unregistered, and we must clear the bit.

Another case is shrinker registration.  We want two things there:

1) do_shrink_slab() can be called only for completely registered
   shrinkers;

2) shrinker internal lists may be populated in any order with
   register_shrinker_prepared() (let's talk on the example with sb).  Both
   of:

  a)list_lru_add(&inode->i_sb->s_inode_lru, &inode->i_lru); [cpu0]
    memcg_set_shrinker_bit();                               [cpu0]
    ...
    register_shrinker_prepared();                           [cpu1]

  and

  b)register_shrinker_prepared();                           [cpu0]
    ...
    list_lru_add(&inode->i_sb->s_inode_lru, &inode->i_lru); [cpu1]
    memcg_set_shrinker_bit();                               [cpu1]

   are legitimate.  We don't want to impose restriction here and to
   force people to use only (b) variant.  We don't want to force people to
   care, there is no elements in LRU lists before the shrinker is
   completely registered.  Internal users of LRU lists and shrinker code
   are two different subsystems, and they have to be closed in themselves
   each other.

In (a) case we have the bit set before shrinker is completely
registered.  We don't want do_shrink_slab() is called at this moment, so
we have to detect such the registering shrinkers.

Before this patch list_empty() (shrinker is not linked to the list)
check was used for that.  So, in (a) there could be a bit set, but we
don't call do_shrink_slab() unless shrinker is linked to the list.  It's
just an indicator, I just overloaded linking to the list.

This was not the best solution, since it's better not to touch the
shrinker memory from shrink_slab_memcg() before it's completely
registered (this also will be useful in the future to make shrink_slab()
completely lockless).

So, this patch introduces better way to detect registering shrinker,
which allows not to dereference shrinker memory.  It's just a ~0UL
value, which we insert into the IDR during ID allocation.  After
shrinker is ready to be used, we insert actual shrinker pointer in the
IDR, and it becomes available to shrink_slab_memcg().

We can't use NULL instead of this new value for this purpose as:
shrink_slab_memcg() already uses NULL to detect unregistered shrinkers,
and we don't want the function sees NULL and clears the bit, otherwise
(a) won't work.

This is the only thing the patch makes: the better way to detect
registering shrinker.  Nothing else this patch makes.

Also this gives a better assembler, but it's minor side of the patch:

Before:
  callq  <idr_find>
  mov    %rax,%r15
  test   %rax,%rax
  je     <shrink_slab_memcg+0x1d5>
  mov    0x20(%rax),%rax
  lea    0x20(%r15),%rdx
  cmp    %rax,%rdx
  je     <shrink_slab_memcg+0xbd>
  mov    0x8(%rsp),%edx
  mov    %r15,%rsi
  lea    0x10(%rsp),%rdi
  callq  <do_shrink_slab>

After:
  callq  <idr_find>
  mov    %rax,%r15
  lea    -0x1(%rax),%rax
  cmp    $0xfffffffffffffffd,%rax
  ja     <shrink_slab_memcg+0x1cd>
  mov    0x8(%rsp),%edx
  mov    %r15,%rsi
  lea    0x10(%rsp),%rdi
  callq  ffffffff810cefd0 <do_shrink_slab>

[ktkhai@virtuozzo.com: add #ifdef CONFIG_MEMCG_KMEM around idr_replace()]
  Link: http://lkml.kernel.org/r/758b8fec-7573-47eb-b26a-7b2847ae7b8c@virtuozzo.com
Link: http://lkml.kernel.org/r/153355467546.11522.4518015068123480218.stgit@localhost.localdomain
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Josef Bacik <jbacik@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -170,6 +170,20 @@ static LIST_HEAD(shrinker_list);
 static DECLARE_RWSEM(shrinker_rwsem);
 
 #ifdef CONFIG_MEMCG_KMEM
+
+/*
+ * We allow subsystems to populate their shrinker-related
+ * LRU lists before register_shrinker_prepared() is called
+ * for the shrinker, since we don't want to impose
+ * restrictions on their internal registration order.
+ * In this case shrink_slab_memcg() may find corresponding
+ * bit is set in the shrinkers map.
+ *
+ * This value is used by the function to detect registering
+ * shrinkers and to skip do_shrink_slab() calls for them.
+ */
+#define SHRINKER_REGISTERING ((struct shrinker *)~0UL)
+
 static DEFINE_IDR(shrinker_idr);
 static int shrinker_nr_max;
 
@@ -179,7 +193,7 @@ static int prealloc_memcg_shrinker(struct shrinker *shrinker)
 
 	down_write(&shrinker_rwsem);
 	/* This may call shrinker, so it must use down_read_trylock() */
-	id = idr_alloc(&shrinker_idr, shrinker, 0, 0, GFP_KERNEL);
+	id = idr_alloc(&shrinker_idr, SHRINKER_REGISTERING, 0, 0, GFP_KERNEL);
 	if (id < 0)
 		goto unlock;
 
@@ -364,21 +378,6 @@ int prealloc_shrinker(struct shrinker *shrinker)
 	if (!shrinker->nr_deferred)
 		return -ENOMEM;
 
-	/*
-	 * There is a window between prealloc_shrinker()
-	 * and register_shrinker_prepared(). We don't want
-	 * to clear bit of a shrinker in such the state
-	 * in shrink_slab_memcg(), since this will impose
-	 * restrictions on a code registering a shrinker
-	 * (they would have to guarantee, their LRU lists
-	 * are empty till shrinker is completely registered).
-	 * So, we differ the situation, when 1)a shrinker
-	 * is semi-registered (id is assigned, but it has
-	 * not yet linked to shrinker_list) and 2)shrinker
-	 * is not registered (id is not assigned).
-	 */
-	INIT_LIST_HEAD(&shrinker->list);
-
 	if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
 		if (prealloc_memcg_shrinker(shrinker))
 			goto free_deferred;
@@ -408,6 +407,9 @@ void register_shrinker_prepared(struct shrinker *shrinker)
 {
 	down_write(&shrinker_rwsem);
 	list_add_tail(&shrinker->list, &shrinker_list);
+#ifdef CONFIG_MEMCG_KMEM
+	idr_replace(&shrinker_idr, shrinker, shrinker->id);
+#endif
 	up_write(&shrinker_rwsem);
 }
 
@@ -589,15 +591,12 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
 		struct shrinker *shrinker;
 
 		shrinker = idr_find(&shrinker_idr, i);
-		if (unlikely(!shrinker)) {
-			clear_bit(i, map->map);
+		if (unlikely(!shrinker || shrinker == SHRINKER_REGISTERING)) {
+			if (!shrinker)
+				clear_bit(i, map->map);
 			continue;
 		}
 
-		/* See comment in prealloc_shrinker() */
-		if (unlikely(list_empty(&shrinker->list)))
-			continue;
-
 		ret = do_shrink_slab(&sc, shrinker, priority);
 		if (ret == SHRINK_EMPTY) {
 			clear_bit(i, map->map);