linux-kernel - Re: [PATCH] xen-blkfront: Use the bitmap API when applicable

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Date:   Thu, 2 Dec 2021 20:07:17 +0100
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Joe Perches <joe@...ches.com>, Juergen Gross <jgross@...e.com>,
        boris.ostrovsky@...cle.com, sstabellini@...nel.org,
        roger.pau@...rix.com, axboe@...nel.dk
Cc:     xen-devel@...ts.xenproject.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] xen-blkfront: Use the bitmap API when applicable

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

new call
--------
end = 68
nr_minors = 128 (from previous call)
--> no need to reallocate

CJ