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

[Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>]

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");