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[Qemu-devel] [PATCH] Remove ill-advised fallback when we cannot allocate

From: David Gibson
Subject: [Qemu-devel] [PATCH] Remove ill-advised fallback when we cannot allocate from -mem-path
Date: Wed, 1 Aug 2012 14:41:05 +1000

When the -mem-path option is specified we try to allocate memory for guest
RAM from mmap()ing files in the given path rather than from anonymous
memory as usual.  If we are unable to allocate memory from the path, we
fall back to anonymous memory, with a rather cryptic error message.

This fallback is a bad idea - if the user specified a backing path, they
did so for a reason and falling back automatically could lead to confusing

Under Book3S-HV KVM on PowerPC the failure is particularly bad, because the
page size of the backing file limits what page sizes can be supported in the
guest.  We have code to automatically advertise the available page sizes to
the guest, but this will give the wrong information in the case of a fall
back to anonymous memory, because by the time we're producing that infor
there's no easy way to determine that such a fallback occurred.

Signed-off-by: David Gibson <address@hidden>
 exec.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/exec.c b/exec.c
index feb4795..544a090 100644
--- a/exec.c
+++ b/exec.c
@@ -2425,7 +2425,6 @@ static void *file_ram_alloc(RAMBlock *block,
     area = mmap(0, memory, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
     if (area == MAP_FAILED) {
-        perror("file_ram_alloc: can't mmap RAM pages");
         return (NULL);
@@ -2528,8 +2527,9 @@ ram_addr_t qemu_ram_alloc_from_ptr(ram_addr_t size, void 
 #if defined (__linux__) && !defined(TARGET_S390X)
             new_block->host = file_ram_alloc(new_block, size, mem_path);
             if (!new_block->host) {
-                new_block->host = qemu_vmalloc(size);
-                qemu_madvise(new_block->host, size, QEMU_MADV_MERGEABLE);
+                fprintf(stderr, "Unable to map %llu bytes of RAM from 
+                        " backing path %s\n", (unsigned long long)size, 
+                exit(1);
             fprintf(stderr, "-mem-path option unsupported\n");

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