Re: [Qemu-devel] [Xen-devel] frequently ballooning results in qemu exit

[Stefano Stabellini <stefano.stabellini@eu.citrix.com>]

On Mon, 25 Mar 2013, Hanweidong wrote:
> We fixed this issue by below patch which computed the correct size for test_bits(). qemu_get_ram_ptr() and qemu_safe_ram_ptr() will call xen_map_cache() with size is 0 if the requested address is in the RAM. Then xen_map_cache() will pass the size 0 to test_bits() for checking if the corresponding pfn was mapped in cache. But test_bits() will always return 1 when size is 0 without any bit testing. Actually, for this case, test_bits should check one bit. So this patch introduced a __test_bit_size which is greater than 0 and a multiple of XC_PAGE_SIZE, then test_bits can work correctly with __test_bit_size >> XC_PAGE_SHIFT as its size.
> 
> Signed-off-by: Zhenguo Wang <wangzhenguo@huawei.com>
> Signed-off-by: Weidong Han <hanweidong@huawei.com>

Thanks for the patch and for debugging this difficult problem.
The reality is that size is never actually 0: when qemu_get_ram_ptr
calls xen_map_cache with size 0, it actually means "map until the end of
the page". As a consequence test_bits should always test at least 1 bit,
like you wrote.

I tried to simplify your patch a bit, does this one work for you?


diff --git a/xen-mapcache.c b/xen-mapcache.c
index dc6d1fa..b03b373 100644
--- a/xen-mapcache.c
+++ b/xen-mapcache.c
@@ -202,6 +202,7 @@ uint8_t *xen_map_cache(hwaddr phys_addr, hwaddr size,
     hwaddr address_index;
     hwaddr address_offset;
     hwaddr __size = size;
+    hwaddr __test_bit_size = size >> XC_PAGE_SHIFT;
     bool translated = false;
 
 tryagain:
@@ -224,12 +225,16 @@ tryagain:
     } else {
         __size = MCACHE_BUCKET_SIZE;
     }
+    /* always test at least one page */
+    if (!__test_bit_size) {
+        __test_bit_size = 1UL;
+    }
 
     entry = &mapcache->entry[address_index % mapcache->nr_buckets];
 
     while (entry && entry->lock && entry->vaddr_base &&
             (entry->paddr_index != address_index || entry->size != __size ||
-             !test_bits(address_offset >> XC_PAGE_SHIFT, size >> XC_PAGE_SHIFT,
+             !test_bits(address_offset >> XC_PAGE_SHIFT, __test_bit_size,
                  entry->valid_mapping))) {
         pentry = entry;
         entry = entry->next;
@@ -241,13 +246,13 @@ tryagain:
     } else if (!entry->lock) {
         if (!entry->vaddr_base || entry->paddr_index != address_index ||
                 entry->size != __size ||
-                !test_bits(address_offset >> XC_PAGE_SHIFT, size >> XC_PAGE_SHIFT,
+                !test_bits(address_offset >> XC_PAGE_SHIFT, __test_bit_size,
                     entry->valid_mapping)) {
             xen_remap_bucket(entry, __size, address_index);
         }
     }
 
-    if(!test_bits(address_offset >> XC_PAGE_SHIFT, size >> XC_PAGE_SHIFT,
+    if(!test_bits(address_offset >> XC_PAGE_SHIFT, __test_bit_size,
                 entry->valid_mapping)) {
         mapcache->last_address_index = -1;
         if (!translated && mapcache->phys_offset_to_gaddr) {