Re: [PATCH v4 5/5] backends/hostmem: Report error when memory size is unaligned

[Mario Casquero]

This patch has been successfully tested. Try to allocate some 2M
hugepages in the host, then boot up a VM with the memory size
unaligned and backed by a file, QEMU prompts the following message:
qemu-system-x86_64: backend 'memory-backend-file' memory size must be
multiple of 2 MiB

Tested-by: Mario Casquero <mcasquer@redhat.com>

On Wed, Jun 5, 2024 at 12:45 PM Michal Privoznik <mprivozn@redhat.com> wrote:
>
> If memory-backend-{file,ram} has a size that's not aligned to
> underlying page size it is not only wasteful, but also may lead
> to hard to debug behaviour. For instance, in case
> memory-backend-file and hugepages, madvise() and mbind() fail.
> Rightfully so, page is the smallest unit they can work with. And
> even though an error is reported, the root cause it not very
> clear:
>
>   qemu-system-x86_64: Couldn't set property 'dump' on 'memory-backend-file': 
> Invalid argument
>
> After this commit:
>
>   qemu-system-x86_64: backend 'memory-backend-file' memory size must be 
> multiple of 2 MiB
>
> Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> Tested-by: Mario Casquero <mcasquer@redhat.com>
> ---
>  backends/hostmem.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/backends/hostmem.c b/backends/hostmem.c
> index 012a8c190f..4d6c69fe4d 100644
> --- a/backends/hostmem.c
> +++ b/backends/hostmem.c
> @@ -20,6 +20,7 @@
>  #include "qom/object_interfaces.h"
>  #include "qemu/mmap-alloc.h"
>  #include "qemu/madvise.h"
> +#include "qemu/cutils.h"
>  #include "hw/qdev-core.h"
>
>  #ifdef CONFIG_NUMA
> @@ -337,6 +338,7 @@ host_memory_backend_memory_complete(UserCreatable *uc, 
> Error **errp)
>      HostMemoryBackendClass *bc = MEMORY_BACKEND_GET_CLASS(uc);
>      void *ptr;
>      uint64_t sz;
> +    size_t pagesize;
>      bool async = !phase_check(PHASE_LATE_BACKENDS_CREATED);
>
>      if (!bc->alloc) {
> @@ -348,6 +350,14 @@ host_memory_backend_memory_complete(UserCreatable *uc, 
> Error **errp)
>
>      ptr = memory_region_get_ram_ptr(&backend->mr);
>      sz = memory_region_size(&backend->mr);
> +    pagesize = qemu_ram_pagesize(backend->mr.ram_block);
> +
> +    if (!QEMU_IS_ALIGNED(sz, pagesize)) {
> +        g_autofree char *pagesize_str = size_to_str(pagesize);
> +        error_setg(errp, "backend '%s' memory size must be multiple of %s",
> +                   object_get_typename(OBJECT(uc)), pagesize_str);
> +        return;
> +    }
>
>      if (backend->merge &&
>          qemu_madvise(ptr, sz, QEMU_MADV_MERGEABLE)) {
> --
> 2.44.1
>
>