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


  • To: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>, Joe Perches <joe@xxxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, sstabellini@xxxxxxxxxx, roger.pau@xxxxxxxxxx, axboe@xxxxxxxxx
  • From: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
  • Date: Fri, 3 Dec 2021 16:04:11 -0500
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  • Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, linux-block@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, kernel-janitors@xxxxxxxxxxxxxxx
  • Delivery-date: Fri, 03 Dec 2021 21:05:00 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>


On 12/3/21 10:54 AM, Christophe JAILLET wrote:
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) {


I don't think this will work anymore, we may now fail if another thread gets a 
minor above the original (i.e. no aligned) @end.


-boris


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");





 


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