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 <mcasq...@redhat.com>
On Wed, Jun 5, 2024 at 12:45 PM Michal Privoznik <mpriv...@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 <mpriv...@redhat.com>
> Reviewed-by: Philippe Mathieu-Daudé <phi...@linaro.org>
> Tested-by: Mario Casquero <mcasq...@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
>
>
