Re: [PATCH 1/3] hostmem: Detect and cache fs type for file hostmem

[David Hildenbrand <david@redhat.com>]

On 19.04.23 00:57, Peter Xu wrote:
> Detect the file system for a memory-backend-file object and cache it within
> the object if possible when CONFIG_LINUX (using statfs).
> 
> Only support the two important types of memory (tmpfs, hugetlbfs) and keep
> the rest as "unknown" for now.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>   backends/hostmem-file.c  | 37 ++++++++++++++++++++++++++++++++++++-
>   include/sysemu/hostmem.h |  1 +
>   2 files changed, 37 insertions(+), 1 deletion(-)
> 
> diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
> index 25141283c4..2484e45a11 100644
> --- a/backends/hostmem-file.c
> +++ b/backends/hostmem-file.c
> @@ -18,13 +18,17 @@
>   #include "sysemu/hostmem.h"
>   #include "qom/object_interfaces.h"
>   #include "qom/object.h"
> +#ifdef CONFIG_LINUX
> +#include <sys/vfs.h>
> +#include <linux/magic.h>
> +#endif
>   
>   OBJECT_DECLARE_SIMPLE_TYPE(HostMemoryBackendFile, MEMORY_BACKEND_FILE)
>   
>   
>   struct HostMemoryBackendFile {
>       HostMemoryBackend parent_obj;
> -
> +    __fsword_t fs_type;
>       char *mem_path;
>       uint64_t align;
>       bool discard_data;
> @@ -52,6 +56,15 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
>           return;
>       }
>   
> +#ifdef CONFIG_LINUX
> +    struct statfs fs;
> +    if (!statfs(fb->mem_path, &fs)) {
> +        fb->fs_type = fs.f_type;
> +    } else {
> +        fb->fs_type = 0;
> +    }
> +#endif
> +


Instead of using statfs, why not implement something like 
qemu_fd_getpagesize(), that also relies on HUGETLBFS_MAGIC already, meaning

	size_t qemu_fd_type(int fd)

which uses fstatfs() instead? As an abstraction, as Daniel suggests, use 
a new enum to return the type -- "0" meaning "unknown".

Then you can even avoid the caching in hostmem code and simply call it 
directly from uffd code.

-- 
Thanks,

David / dhildenb