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

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Message-ID: <863f2cddacac590d581cda09d548ee0a652df8a1.camel@perches.com>
Date:   Thu, 02 Dec 2021 19:03:32 -0800
From:   Joe Perches <joe@...ches.com>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        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

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.

> 
> 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