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Subject[PATCH] mm: Use special value SHRINKER_REGISTERING instead list_empty() check
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The patch introduces a special value SHRINKER_REGISTERING to use instead
of list_empty() to detect a semi-registered shrinker.

This should be clearer for a reader since "list is empty" is not
an intuitive state of a shrinker), and this gives a better assembler
code:

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>

Also, improve the comment.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
---
mm/vmscan.c | 42 ++++++++++++++++++++----------------------
1 file changed, 20 insertions(+), 22 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 0d980e801b8a..c18c4acf9599 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -170,6 +170,21 @@ static LIST_HEAD(shrinker_list);
static DECLARE_RWSEM(shrinker_rwsem);

#ifdef CONFIG_MEMCG_KMEM
+
+/*
+ * 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 use this value to detect the situation,
+ * when id is assigned, but shrinker is not completely
+ * registered yet.
+ */
+#define SHRINKER_REGISTERING ((struct shrinker *)~0UL)
+
static DEFINE_IDR(shrinker_idr);
static int shrinker_nr_max;

@@ -179,7 +194,7 @@ 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 +379,6 @@ 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 +408,7 @@ void register_shrinker_prepared(struct shrinker *shrinker)
{
down_write(&shrinker_rwsem);
list_add_tail(&shrinker->list, &shrinker_list);
+ idr_replace(&shrinker_idr, shrinker, shrinker->id);
up_write(&shrinker_rwsem);
}

@@ -589,15 +590,12 @@ 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);
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