[PATCH] xen/page_alloc: Remove dead code in alloc_domheap_pages()

[Julien Grall <julien@xen.org>]

From: Julien Grall <jgrall@amazon.com>

Since commit 1aac966e24e9 "xen: support RAM at addresses 0 and 4096",
bits_to_zone() will never return 0 and it is expected that we have
minimum 2 zones.

Therefore the check in alloc_domheap_pages() is unnecessary and can
be removed. However, for sanity, it is replaced with an ASSERT().

Also take the opportunity to check atbuild time that NR_ZONES is minimum
2.

This bug was discovered and resolved using Coverity Static Analysis
Security Testing (SAST) by Synopsys, Inc.

Signed-off-by: Julien Grall <jgrall@amazon.com>
---
 xen/common/page_alloc.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 1744e6faa5c4..68e47d963842 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -457,6 +457,12 @@ static long total_avail_pages;
 static DEFINE_SPINLOCK(heap_lock);
 static long outstanding_claims; /* total outstanding claims by all domains */
 
+static void __init __maybe_unused build_assertions(void)
+{
+    /* Zone 0 is reserved for Xen, so we at least need two zones to function.*/
+    BUILD_BUG_ON(NR_ZONES < 2);
+}
+
 unsigned long domain_adjust_tot_pages(struct domain *d, long pages)
 {
     long dom_before, dom_after, dom_claimed, sys_before, sys_after;
@@ -2340,8 +2346,9 @@ struct page_info *alloc_domheap_pages(
 
     bits = domain_clamp_alloc_bitsize(memflags & MEMF_no_owner ? NULL : d,
                                       bits ? : (BITS_PER_LONG+PAGE_SHIFT));
-    if ( (zone_hi = min_t(unsigned int, bits_to_zone(bits), zone_hi)) == 0 )
-        return NULL;
+
+    zone_hi = min_t(unsigned int, bits_to_zone(bits), zone_hi);
+    ASSERT(zone_hi != 0);
 
     if ( memflags & MEMF_no_owner )
         memflags |= MEMF_no_refcount;
-- 
2.17.1