Re: [Qemu-devel] [PATCH v11 2/3] util/mmap-alloc: support MAP_SYNC in qemu_ram_mmap()

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jan 29, 2019 at 10:49:09PM +0800, Zhang, Yi wrote:
> From: Zhang Yi <yi.z.zhang@linux.intel.com>
> 
> When a file supporting DAX is used as vNVDIMM backend, mmap it with
> MAP_SYNC flag in addition which can ensure file system metadata
> synced in each guest writes to the backend file, without other QEMU
> actions (e.g., periodic fsync() by QEMU).
> 
> Current, We have below different possible use cases:
> 
> 1. pmem=on is set, shared=on is set, MAP_SYNC supported:
>    a: backend is a dax supporting file.
>     - MAP_SYNC will active.
>    b: backend is not a dax supporting file.
>     - mmap will trigger a warning. then MAP_SYNC flag will be ignored
> 
> 2. The rest of cases:
>    - we will never pass the MAP_SYNC to mmap2
> 
> Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> Signed-off-by: Zhang Yi <yi.z.zhang@linux.intel.com>
> ---
>  include/qemu/osdep.h | 21 +++++++++++++++++++++
>  util/mmap-alloc.c    | 28 +++++++++++++++++++++++++++-
>  2 files changed, 48 insertions(+), 1 deletion(-)
> 
> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> index 457d24e..96209bb 100644
> --- a/include/qemu/osdep.h
> +++ b/include/qemu/osdep.h
> @@ -419,6 +419,27 @@ void qemu_anon_ram_free(void *ptr, size_t size);
>  #  define QEMU_VMALLOC_ALIGN getpagesize()
>  #endif
>  
> +/*
> + * MAP_SHARED_VALIDATE and MAP_SYNC are introduced in Linux kernel
> + * 4.15, so they may not be defined when compiling on older kernels.
> + */
> +#ifdef CONFIG_LINUX
> +
> +#include <linux/mman.h>
> +
> +#ifndef MAP_SYNC
> +#define MAP_SYNC 0x80000
> +#endif
> +
> +#ifndef MAP_SHARED_VALIDATE
> +#define MAP_SHARED_VALIDATE 0x03
> +#endif
> +

I commented on this part in v7. That's a wrong way to handle
compatibility.

We had this discussion several times in the past. commit log doesn't
mention any reasons to ignore this.  All for setting a single bit in a
single system call.  This is getting discouraging.


> +#else  /* !CONFIG_LINUX */
> +#define MAP_SYNC              0x0
> +#define MAP_SHARED_VALIDATE   0x0
> +#endif /* CONFIG_LINUX */
> +
>  #ifdef CONFIG_POSIX
>  struct qemu_signalfd_siginfo {
>      uint32_t ssi_signo;   /* Signal number */
> diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
> index 97bbeed..2c86ad2 100644
> --- a/util/mmap-alloc.c
> +++ b/util/mmap-alloc.c
> @@ -101,6 +101,7 @@ void *qemu_ram_mmap(int fd,
>  #else
>      void *ptr = mmap(0, total, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
>  #endif
> +    int mmap_xflags = 0;

That's not a good variable name. It's extra for you since
you are writing the patch but it makes no sense
in the context of the function. Just mmap_flags
will do - and I would put all flags there, not just
the "extra" that this patch is adding.


>      size_t offset;
>      void *ptr1;
>  
> @@ -111,13 +112,38 @@ void *qemu_ram_mmap(int fd,
>      assert(is_power_of_2(align));
>      /* Always align to host page size */
>      assert(align >= getpagesize());
> +    if (shared && is_pmem) {
> +        mmap_xflags = MAP_SYNC | MAP_SHARED_VALIDATE;
> +    }
>  
>      offset = QEMU_ALIGN_UP((uintptr_t)ptr, align) - (uintptr_t)ptr;
> +retry_mmap:
>      ptr1 = mmap(ptr + offset, size, PROT_READ | PROT_WRITE,
>                  MAP_FIXED |
>                  (fd == -1 ? MAP_ANONYMOUS : 0) |
> -                (shared ? MAP_SHARED : MAP_PRIVATE),
> +                (shared ? MAP_SHARED : MAP_PRIVATE) | mmap_xflags,
>                  fd, 0);
> +
> +    /* if map failed with MAP_SHARED_VALIDATE | MAP_SYNC,
> +     * we try with MAP_SHARED_VALIDATE without MAP_SYNC
> +     */
> +    if (ptr1 == MAP_FAILED &&
> +        mmap_xflags == (MAP_SYNC | MAP_SHARED_VALIDATE)) {
> +        if (errno == ENOTSUP) {
> +            perror("failed to validate with mapping flags");
> +        }
> +        mmap_xflags = MAP_SHARED_VALIDATE;
> +        goto retry_mmap;

Have you read
https://homepages.cwi.nl/~storm/teaching/reader/Dijkstra68.pdf

Please just call the function twice. You don't need goto
to repeat a single line.


> +    }
> +    /* MAP_SHARED_VALIDATE flag is available since Linux 4.15
> +     * Test only with MAP_SHARED_VALIDATE flag for compatibility.
> +     * Then ignore the MAP_SHARED_VALIDATE flag and retry again
> +     */
> +    if (mmap_xflags == MAP_SHARED_VALIDATE &&
> +        ptr1 == MAP_FAILED) {
> +            mmap_xflags &= ~MAP_SHARED_VALIDATE;
> +            goto retry_mmap;
> +    }
>      if (ptr1 == MAP_FAILED) {
>          munmap(ptr, total);
>          return MAP_FAILED;
> -- 
> 2.7.4