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Subject[PATCH v19 2/7] xbitmap: potential improvement
Date
This patch made some changes to the original xbitmap implementation from
the linux-dax tree:

- remove xb_fill() and xb_zero() from xbitmap.h since they are not
implemented;

- xb_test_bit: changed "ebit > BITS_PER_LONG" to "ebit >= BITS_PER_LONG",
because bit 64 beyonds the "unsigned long" exceptional entry (0 to 63);

- xb_set_bit: delete the new inserted radix_tree_node when failing to
get the per cpu ida bitmap, this avoids the kind of memory leak of the
unused radix tree node left in the tree.

- xb_clear_bit: change it to be a void function, since the original
implementation reurns nothing than a 0.

- remove the comment above "#define XB_INDEX_BITS", because it causes
confusion based on the feedbacks from the previous discussion;

- xb_preload: with the original implementation, the CPU that successfully
do __radix_tree_preload() may get into sleep by kmalloc(), which has a
risk of getting the caller of xb_preload() scheduled to another CPU
after waken up, and the new CPU may not have radix_tree_node
pre-allocated there, this will be a problem when inserting a node to
the tree later. This patch moves __radix_tree_preload() after kmalloc()
and returns a boolean to indicate the success or failure. Also, add the
__must_check annotation to xb_preload for prudence purpose.

Signed-off-by: Wei Wang <wei.w.wang@intel.com>
Cc: Matthew Wilcox <mawilcox@microsoft.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
include/linux/xbitmap.h | 5 +----
lib/radix-tree.c | 27 +++++++++++++++++++++------
lib/xbitmap.c | 24 +++++++++++++-----------
3 files changed, 35 insertions(+), 21 deletions(-)

diff --git a/include/linux/xbitmap.h b/include/linux/xbitmap.h
index ed75d87..b4d8375 100644
--- a/include/linux/xbitmap.h
+++ b/include/linux/xbitmap.h
@@ -36,15 +36,12 @@ int xb_set_bit(struct xb *xb, unsigned long bit);
bool xb_test_bit(const struct xb *xb, unsigned long bit);
int xb_clear_bit(struct xb *xb, unsigned long bit);

-int xb_zero(struct xb *xb, unsigned long start, unsigned long nbits);
-int xb_fill(struct xb *xb, unsigned long start, unsigned long nbits);
-
static inline bool xb_empty(const struct xb *xb)
{
return radix_tree_empty(&xb->xbrt);
}

-void xb_preload(gfp_t);
+bool xb_preload(gfp_t);

static inline void xb_preload_end(void)
{
diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index 7000ad6..a039588 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -77,9 +77,6 @@ static struct kmem_cache *radix_tree_node_cachep;
RADIX_TREE_MAP_SHIFT))
#define IDA_PRELOAD_SIZE (IDA_MAX_PATH * 2 - 1)

-/*
- * The XB can go up to unsigned long, but also uses a bitmap.
- */
#define XB_INDEX_BITS (BITS_PER_LONG - ilog2(IDA_BITMAP_BITS))
#define XB_MAX_PATH (DIV_ROUND_UP(XB_INDEX_BITS, \
RADIX_TREE_MAP_SHIFT))
@@ -2145,17 +2142,35 @@ int ida_pre_get(struct ida *ida, gfp_t gfp)
}
EXPORT_SYMBOL(ida_pre_get);

-void xb_preload(gfp_t gfp)
+/**
+ * xb_preload - preload for xb_set_bit()
+ * @gfp_mask: allocation mask to use for preloading
+ *
+ * Preallocate memory to use for the next call to xb_set_bit(). On success,
+ * return true, with preemption disabled. On error, return false with
+ * preemption not disabled.
+ */
+__must_check bool xb_preload(gfp_t gfp)
{
- __radix_tree_preload(gfp, XB_PRELOAD_SIZE);
if (!this_cpu_read(ida_bitmap)) {
struct ida_bitmap *bitmap = kmalloc(sizeof(*bitmap), gfp);

if (!bitmap)
- return;
+ return false;
+ /*
+ * The per-CPU variable is updated with preemption enabled.
+ * If the calling task is unlucky to be scheduled to another
+ * CPU which has no ida_bitmap allocation, it will be detected
+ * when setting a bit (i.e. __xb_set_bit()).
+ */
bitmap = this_cpu_cmpxchg(ida_bitmap, NULL, bitmap);
kfree(bitmap);
}
+
+ if (__radix_tree_preload(gfp, XB_PRELOAD_SIZE) < 0)
+ return false;
+
+ return true;
}
EXPORT_SYMBOL(xb_preload);

diff --git a/lib/xbitmap.c b/lib/xbitmap.c
index 2b547a73..182aa29 100644
--- a/lib/xbitmap.c
+++ b/lib/xbitmap.c
@@ -39,8 +39,10 @@ int xb_set_bit(struct xb *xb, unsigned long bit)
return 0;
}
bitmap = this_cpu_xchg(ida_bitmap, NULL);
- if (!bitmap)
+ if (!bitmap) {
+ __radix_tree_delete(root, node, slot);
return -EAGAIN;
+ }
memset(bitmap, 0, sizeof(*bitmap));
bitmap->bitmap[0] = tmp >> RADIX_TREE_EXCEPTIONAL_SHIFT;
rcu_assign_pointer(*slot, bitmap);
@@ -54,8 +56,10 @@ int xb_set_bit(struct xb *xb, unsigned long bit)
return 0;
}
bitmap = this_cpu_xchg(ida_bitmap, NULL);
- if (!bitmap)
+ if (!bitmap) {
+ __radix_tree_delete(root, node, slot);
return -EAGAIN;
+ }
memset(bitmap, 0, sizeof(*bitmap));
__radix_tree_replace(root, node, slot, bitmap, NULL);
}
@@ -73,7 +77,7 @@ EXPORT_SYMBOL(xb_set_bit);
* This function is used to clear a bit in the xbitmap. If all the bits of the
* bitmap are 0, the bitmap will be freed.
*/
-int xb_clear_bit(struct xb *xb, unsigned long bit)
+void xb_clear_bit(struct xb *xb, unsigned long bit)
{
unsigned long index = bit / IDA_BITMAP_BITS;
struct radix_tree_root *root = &xb->xbrt;
@@ -90,25 +94,23 @@ int xb_clear_bit(struct xb *xb, unsigned long bit)
unsigned long tmp = (unsigned long)bitmap;

if (ebit >= BITS_PER_LONG)
- return 0;
+ return;
tmp &= ~(1UL << ebit);
if (tmp == RADIX_TREE_EXCEPTIONAL_ENTRY)
__radix_tree_delete(root, node, slot);
else
rcu_assign_pointer(*slot, (void *)tmp);
- return 0;
+ return;
}

if (!bitmap)
- return 0;
+ return;

__clear_bit(bit, bitmap->bitmap);
if (bitmap_empty(bitmap->bitmap, IDA_BITMAP_BITS)) {
kfree(bitmap);
__radix_tree_delete(root, node, slot);
}
-
- return 0;
}
EXPORT_SYMBOL(xb_clear_bit);

@@ -133,7 +135,7 @@ bool xb_test_bit(const struct xb *xb, unsigned long bit)
return false;
if (radix_tree_exception(bitmap)) {
bit += RADIX_TREE_EXCEPTIONAL_SHIFT;
- if (bit > BITS_PER_LONG)
+ if (bit >= BITS_PER_LONG)
return false;
return (unsigned long)bitmap & (1UL << bit);
}
@@ -151,9 +153,9 @@ void xbitmap_check_bit(unsigned long bit)
assert(!xb_test_bit(&xb1, bit));
assert(xb_set_bit(&xb1, bit) == 0);
assert(xb_test_bit(&xb1, bit));
- assert(xb_clear_bit(&xb1, bit) == 0);
+ xb_clear_bit(&xb1, bit);
assert(xb_empty(&xb1));
- assert(xb_clear_bit(&xb1, bit) == 0);
+ xb_clear_bit(&xb1, bit);
assert(xb_empty(&xb1));
xb_preload_end();
}
--
2.7.4
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