[Xen-devel] [PATCH 1/3] xmalloc: stop using a magic '1' in alignment padding

[Paul Durrant <paul.durrant@xxxxxxxxxx>]

Alignment padding inserts a pseudo block header in front of the allocation,
sets its size field to the pad size and then ORs in 1, which is equivalent
to marking it as a free block, so that xfree() can distinguish it from a
real block header.

This patch simply replaces the magic '1' with the defined 'FREE_BLOCK' to
make it more obvious what's going on. Also, whilst in the neighbourhood,
it removes a stray space after a cast.

Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
---
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Julien Grall <julien.grall@xxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Tim Deegan <tim@xxxxxxx>
Cc: Wei Liu <wl@xxxxxxx>
---
 xen/common/xmalloc_tlsf.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/xen/common/xmalloc_tlsf.c b/xen/common/xmalloc_tlsf.c
index 2076953ac4..6d889b7bdc 100644
--- a/xen/common/xmalloc_tlsf.c
+++ b/xen/common/xmalloc_tlsf.c
@@ -595,7 +595,7 @@ void *_xmalloc(unsigned long size, unsigned long align)
         char *q = (char *)p + pad;
         struct bhdr *b = (struct bhdr *)(q - BHDR_OVERHEAD);
         ASSERT(q > (char *)p);
-        b->size = pad | 1;
+        b->size = pad | FREE_BLOCK;
         p = q;
     }
 
@@ -638,12 +638,12 @@ void xfree(void *p)
     }
 
     /* Strip alignment padding. */
-    b = (struct bhdr *)((char *) p - BHDR_OVERHEAD);
-    if ( b->size & 1 )
+    b = (struct bhdr *)((char *)p - BHDR_OVERHEAD);
+    if ( b->size & FREE_BLOCK )
     {
         p = (char *)p - (b->size & ~1u);
         b = (struct bhdr *)((char *)p - BHDR_OVERHEAD);
-        ASSERT(!(b->size & 1));
+        ASSERT(!(b->size & FREE_BLOCK));
     }
 
     xmem_pool_free(p, xenpool);
-- 
2.20.1.2.gb21ebb671


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel