Re: [Qemu-devel] [PATCH V7 3/6] util/mmap-alloc: support MAP_SYNC in qemu_ram_mmap()

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

On Tue, Dec 18, 2018 at 04:17:12PM +0800, Zhang Yi wrote:
> When a file supporting DAX is used as vNVDIMM backend, mmap it with
> MAP_SYNC flag in addition can guarantee the persistence of guest write
> to the backend file without other QEMU actions (e.g., periodic fsync()
> by QEMU).
> 
> A set of RAM_SYNC flags are added to qemu_ram_mmap():
> 
> Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> Signed-off-by: Zhang Yi <yi.z.zhang@linux.intel.com>

Should we be reusing RAM_ flags? These are used widely
already -are you going to allow RAM_SYNC everywhere
where other RAM flags are legal?
E.g. memory_region_init_ram_from_file?


> ---
>  exec.c                    |  2 +-
>  include/exec/memory.h     |  3 +++
>  include/exec/ram_addr.h   |  1 +
>  include/qemu/mmap-alloc.h |  1 +
>  include/qemu/osdep.h      | 29 +++++++++++++++++++++++++++++
>  util/mmap-alloc.c         | 14 ++++++++++----
>  6 files changed, 45 insertions(+), 5 deletions(-)
> 
> diff --git a/exec.c b/exec.c
> index e92a7da..dc4d180 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -2241,7 +2241,7 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
>      int64_t file_size;
>  
>      /* Just support these ram flags by now. */
> -    assert((ram_flags & ~(RAM_SHARED | RAM_PMEM)) == 0);
> +    assert((ram_flags & ~(RAM_SHARED | RAM_PMEM | RAM_SYNC)) == 0);
>  
>      if (xen_enabled()) {
>          error_setg(errp, "-mem-path not supported with Xen");
> diff --git a/include/exec/memory.h b/include/exec/memory.h
> index 667466b..33a4e2c 100644
> --- a/include/exec/memory.h
> +++ b/include/exec/memory.h
> @@ -126,6 +126,9 @@ typedef struct IOMMUNotifier IOMMUNotifier;
>  /* RAM is a persistent kind memory */
>  #define RAM_PMEM (1 << 5)
>  
> +/* RAM can be mmap by a MAP_SYNC flag */

"RAM is mmap-ed with MAP_SYNC"

> +#define RAM_SYNC (1 << 6)
> +
>  static inline void iommu_notifier_init(IOMMUNotifier *n, IOMMUNotify fn,
>                                         IOMMUNotifierFlag flags,
>                                         hwaddr start, hwaddr end,





> diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
> index 9ecd911..d239ce7 100644
> --- a/include/exec/ram_addr.h
> +++ b/include/exec/ram_addr.h
> @@ -87,6 +87,7 @@ long qemu_getrampagesize(void);
>   *              or bit-or of following values
>   *              - RAM_SHARED: mmap the backing file or device with MAP_SHARED
>   *              - RAM_PMEM: the backend @mem_path or @fd is persistent memory
> + *              - RAM_SYNC:   mmap with MAP_SYNC flag
>   *              Other bits are ignored.
>   *  @mem_path or @fd: specify the backing file or device
>   *  @errp: pointer to Error*, to store an error if it happens
> diff --git a/include/qemu/mmap-alloc.h b/include/qemu/mmap-alloc.h
> index 6fe6ed4..1755a8b 100644
> --- a/include/qemu/mmap-alloc.h
> +++ b/include/qemu/mmap-alloc.h
> @@ -18,6 +18,7 @@ size_t qemu_mempath_getpagesize(const char *mem_path);
>   *  @flags: specifies additional properties of the mapping, which can be one or
>   *          bit-or of following values
>   *          - RAM_SHARED: mmap with MAP_SHARED flag
> + *          - RAM_SYNC:   mmap with MAP_SYNC flag
>   *          Other bits are ignored.
>   *
>   * Return:
> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> index 3bf48bc..f94ea68 100644
> --- a/include/qemu/osdep.h
> +++ b/include/qemu/osdep.h
> @@ -410,6 +410,35 @@ 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.

Then you want to import the relevant headers into linux-headers
using scripts/update-linux-headers.sh. Pls do not duplicate code.

> + */
> +#ifdef CONFIG_LINUX
> +
> +#include <sys/mman.h>
> +
> +#ifndef MAP_SHARED_VALIDATE
> +#define MAP_SHARED_VALIDATE   0x3
> +#endif
> +
> +#ifndef MAP_SYNC
> +#define MAP_SYNC              0x80000
> +#endif
> +
> +/* MAP_SYNC is only available with MAP_SHARED_VALIDATE. */
> +#define MAP_SYNC_FLAGS (MAP_SYNC | MAP_SHARED_VALIDATE)

Given mmap flags tend to start with MAP_, adding our own ones
unconditionally is not a good idea.


> +
> +#else  /* !CONFIG_LINUX */
> +
> +#define MAP_SHARED_VALIDATE   0x0
> +#define MAP_SYNC              0x0
> +
> +#define QEMU_HAS_MAP_SYNC     false

Isn't just checking MAP_SYNC enough?

> +#define MAP_SYNC_FLAGS 0
> +
> +#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 8f0a740..89ae862 100644
> --- a/util/mmap-alloc.c
> +++ b/util/mmap-alloc.c
> @@ -99,6 +99,8 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, uint32_t flags)
>      void *ptr = mmap(0, total, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
>  #endif
>      bool shared = flags & RAM_SHARED;
> +    bool is_pmem = flags & RAM_PMEM;
> +    int mmap_xflags = 0;
>      size_t offset;
>      void *ptr1;
>  
> @@ -109,16 +111,20 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, uint32_t flags)
>      assert(is_power_of_2(align));
>      /* Always align to host page size */
>      assert(align >= getpagesize());
> +    if ((flags & RAM_SYNC) && shared && is_pmem) {
> +        mmap_xflags |= MAP_SYNC_FLAGS;
> +    }
>  
>      offset = QEMU_ALIGN_UP((uintptr_t)ptr, align) - (uintptr_t)ptr;
> + retry_mmap_fd:
>      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 (ptr1 == MAP_FAILED) {
> -        munmap(ptr, total);
> -        return MAP_FAILED;
> +    if ((ptr1 == MAP_FAILED) && (mmap_xflags & MAP_SYNC_FLAGS)) {
> +        mmap_xflags &= ~MAP_SYNC_FLAGS;
> +        goto retry_mmap_fd;
>      }
>  
>      if (offset > 0) {
> -- 
> 2.7.4