lkml.org 
[lkml]   [2021]   [Dec]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
SubjectRe: [PATCH] xen-blkfront: Use the bitmap API when applicable
From
Date
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");

\
 
 \ /
  Last update: 2021-12-03 16:54    [W:0.085 / U:0.340 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site