Re: [Qemu-devel] [PATCH v3] migration: initialize RAM to zero

[Anthony Liguori <aliguori@us.ibm.com>]

Paolo Bonzini <pbonzini@redhat.com> writes:

> Using qemu_memalign only leaves the RAM zero by chance, because libc
> will usually use mmap to satisfy our huge requests.  But memory will
> not be zero when using MALLOC_PERTURB_ with a nonzero value.  In the
> case of incoming migration, this breaks a recently-introduced
> invariant (commit f1c7279, migration: do not sent zero pages in
> bulk stage, 2013-03-26).
>
> To fix this, use mmap ourselves to get a well-aligned, always zero
> block for the RAM.  Mmap-ed memory is easy to "trim" at the sides.
>
> This also removes the need to do something special on valgrind
> (see commit c2a8238a, Support running QEMU on Valgrind, 2011-10-31),
> thus effectively reverts that patch.
>
> Reviewed-by: Juan Quintela <quintela@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> 	v2->v3: use MAP_FAILED.  You learn something every day.
>
>  util/oslib-posix.c | 35 ++++++++++++++++++-----------------
>  1 file changed, 18 insertions(+), 17 deletions(-)
>
> diff --git a/util/oslib-posix.c b/util/oslib-posix.c
> index 4e4b819..bda62c0 100644
> --- a/util/oslib-posix.c
> +++ b/util/oslib-posix.c
> @@ -40,7 +40,6 @@ extern int daemon(int, int);
>        Valgrind does not support alignments larger than 1 MiB,
>        therefore we need special code which handles running on Valgrind. */
>  #  define QEMU_VMALLOC_ALIGN (512 * 4096)
> -#  define CONFIG_VALGRIND
>  #elif defined(__linux__) && defined(__s390x__)
>     /* Use 1 MiB (segment size) alignment so gmap can be used by KVM. */
>  #  define QEMU_VMALLOC_ALIGN (256 * 4096)
> @@ -52,12 +51,8 @@ extern int daemon(int, int);
>  #include "sysemu/sysemu.h"
>  #include "trace.h"
>  #include "qemu/sockets.h"
> +#include <sys/mman.h>
>  
> -#if defined(CONFIG_VALGRIND)
> -static int running_on_valgrind = -1;
> -#else
> -#  define running_on_valgrind 0
> -#endif
>  #ifdef CONFIG_LINUX
>  #include <sys/syscall.h>
>  #endif
> @@ -108,22 +103,28 @@ void *qemu_memalign(size_t alignment, size_t size)
>  /* alloc shared memory pages */
>  void *qemu_vmalloc(size_t size)
>  {
> -    void *ptr;
>      size_t align = QEMU_VMALLOC_ALIGN;
> +    size_t total = size + align - getpagesize();
> +    void *ptr = mmap(0, total, PROT_READ | PROT_WRITE,
> +                     MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
> +    size_t offset = QEMU_ALIGN_UP((uintptr_t)ptr, align) - (uintptr_t)ptr;
>  
> -#if defined(CONFIG_VALGRIND)
> -    if (running_on_valgrind < 0) {
> -        /* First call, test whether we are running on Valgrind.
> -           This is a substitute for RUNNING_ON_VALGRIND from valgrind.h. */
> -        const char *ld = getenv("LD_PRELOAD");
> -        running_on_valgrind = (ld != NULL && strstr(ld, "vgpreload"));
> +    if (ptr == MAP_FAILED) {
> +        fprintf(stderr, "Failed to allocate %zu B: %s\n",
> +                size, strerror(errno));
> +        abort();
>      }
> -#endif
>  
> -    if (size < align || running_on_valgrind) {
> -        align = getpagesize();
> +    ptr += offset;
> +    total -= offset;
> +
> +    if (offset > 0) {
> +        munmap(ptr - offset, offset);
>      }
> -    ptr = qemu_memalign(align, size);


Hrm, so we switch from qemu_memalign to mmap() but then we don't modify
qemu_vfree() to do a munmap() over free().

qemu_vfree() doesn't know the size so calling munmap() is tricky.

Regards,

Anthony Liguori

> +    if (total > size) {
> +        munmap(ptr + size, total - size);
> +    }
> +
>      trace_qemu_vmalloc(size, ptr);
>      return ptr;
>  }
> -- 
> 1.8.1.4