Re: [PATCH] xen-blkfront: Use the bitmap API when applicable
From: Christophe JAILLET
Date: Fri Dec 03 2021 - 10:54:17 EST
Le 03/12/2021 à 04:03, Joe Perches a écrit :
On Thu, 2021-12-02 at 20:07 +0100, Christophe JAILLET wrote:
Le 02/12/2021 à 19:16, Joe Perches a écrit :
On Thu, 2021-12-02 at 19:12 +0100, Christophe JAILLET wrote:
Le 02/12/2021 à 07:12, Juergen Gross a écrit :
On 01.12.21 22:10, Christophe JAILLET wrote:
Use 'bitmap_zalloc()' to simplify code, improve the semantic and avoid
some open-coded arithmetic in allocator arguments.
Also change the corresponding 'kfree()' into 'bitmap_free()' to keep
consistency.
Use 'bitmap_copy()' to avoid an explicit 'memcpy()'
[]
diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
[]
@@ -442,16 +442,14 @@ static int xlbd_reserve_minors(unsigned int
minor, unsigned int nr)
if (end > nr_minors) {
unsigned long *bitmap, *old;
- bitmap = kcalloc(BITS_TO_LONGS(end), sizeof(*bitmap),
- GFP_KERNEL);
+ bitmap = bitmap_zalloc(end, GFP_KERNEL);
if (bitmap == NULL)
return -ENOMEM;
spin_lock(&minor_lock);
if (end > nr_minors) {
old = minors;
- memcpy(bitmap, minors,
- BITS_TO_LONGS(nr_minors) * sizeof(*bitmap));
+ bitmap_copy(bitmap, minors, nr_minors);
minors = bitmap;
nr_minors = BITS_TO_LONGS(end) * BITS_PER_LONG;
nr_minors = end;
?
No,
My understanding of the code is that if we lack space (end > nr_minors),
we need to allocate more. In such a case, we want to keep track of what
we have allocated, not what we needed.
The "padding" bits in the "long align" allocation, can be used later.
first call
----------
end = 65
nr_minors = 63
--> we need some space
--> we allocate 2 longs = 128 bits
--> we now use 65 bits of these 128 bits
or 96, 32 or 64 bit longs remember.
32 and 64 for sure, but I was not aware of 96. On which arch?
new call
--------
end = 68
nr_minors = 128 (from previous call)
The initial allocation is now bitmap_zalloc which
specifies only bits and the nr_minors is then in
BITS_TO_LONGS(bits) * BITS_PER_LONG
Perhaps that assumes too much about the internal
implementation of bitmap_alloc
I get your point now, and I agree with you.
Maybe something as what is done in mc-entity.c?
Explicitly require more bits (which will be allocated anyway), instead
of taking advantage (read "hoping") that it will be done.
Could be:
@@ -440,26 +440,25 @@ static int xlbd_reserve_minors(unsigned int minor,
unsigned int nr)
int rc;
if (end > nr_minors) {
unsigned long *bitmap, *old;
- bitmap = kcalloc(BITS_TO_LONGS(end), sizeof(*bitmap),
- GFP_KERNEL);
+ end = ALIGN(end, BITS_PER_LONG);
+ bitmap = bitmap_zalloc(end, GFP_KERNEL);
if (bitmap == NULL)
return -ENOMEM;
spin_lock(&minor_lock);
if (end > nr_minors) {
old = minors;
- memcpy(bitmap, minors,
- BITS_TO_LONGS(nr_minors) * sizeof(*bitmap));
+ bitmap_copy(bitmap, minors, nr_minors);
minors = bitmap;
- nr_minors = BITS_TO_LONGS(end) * BITS_PER_LONG;
+ nr_minors = end;
} else
old = bitmap;
spin_unlock(&minor_lock);
- kfree(old);
+ bitmap_free(old);
}
spin_lock(&minor_lock);
if (find_next_bit(minors, end, minor) >= end) {
bitmap_set(minors, minor, nr);
@@ -2608,11 +2607,11 @@ static void __exit xlblk_exit(void)
{
cancel_delayed_work_sync(&blkfront_work);
xenbus_unregister_driver(&blkfront_driver);
unregister_blkdev(XENVBD_MAJOR, DEV_NAME);
- kfree(minors);
+ bitmap_free(minors);
}
module_exit(xlblk_exit);
MODULE_DESCRIPTION("Xen virtual block device frontend");
MODULE_LICENSE("GPL");