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

[Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>]

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