Re: [Qemu-devel] [PATCH V8 5/5] hostmem-file: add 'sync' option

[Eduardo Habkost]

On Wed, Jan 02, 2019 at 01:26:34PM +0800, Zhang Yi wrote:
> This option controls will mmap the memory backend file with MAP_SYNC flag,
> which can ensure filesystem metadata consistent even after a system crash
> or power failure, if MAP_SYNC flag is supported by the host kernel(Linux
> kernel 4.15 and later) and the backend is a file supporting DAX (e.g.,
> file on ext4/xfs file system mounted with '-o dax').
> 
> It can take one of following values:
>  - on:  try to pass MAP_SYNC to mmap(2); if MAP_SYNC is not supported or
>         'share=off' or 'pmem!=on', QEMU will not pass this flags to
>       mmap(2)
>  - off: default, never pass MAP_SYNC to mmap(2)
> 
> Signed-off-by: Haozhong Zhang <address@hidden>
> Signed-off-by: Zhang Yi <address@hidden>
> ---
[...]
> +vNVDIMM is designed and implemented to guarantee the guest data
> +persistence on the backends even on the host crash and power
> +failures. However, there are still some requirements and limitations
> +as explained below.
> +
>  Though QEMU supports multiple types of vNVDIMM backends on Linux,
> -currently the only one that can guarantee the guest write persistence
> +if MAP_SYNC is not supported by the host kernel and the backends,
> +the only backend that can guarantee the guest write persistence
>  is the device DAX on the real NVDIMM device (e.g., /dev/dax0.0), to
>  which all guest access do not involve any host-side kernel cache.
>  
> +mmap(2) flag MAP_SYNC is added since Linux kernel 4.15. On such
> +systems, QEMU can mmap(2) the backend with MAP_SYNC, which can ensure
> +filesystem metadata consistent even after a system crash or power
> +failure. Besides the host kernel support, enabling MAP_SYNC in QEMU
> +also requires:
> +
> + - the backend is a file supporting DAX, e.g., a file on an ext4 or
> +   xfs file system mounted with '-o dax',
> +
> + - 'sync' option of memory-backend-file is on, and
> +
> + - 'share' option of memory-backend-file is 'on'.
> +
> + - 'pmem' option of memory-backend-file is 'on'

I miss one piece of information here: are there any negative
side-effects of enabling MAP_SYNC on a pmem=on backend?  Could it
affect performance?  If it has no negative effects, why don't we
try to always enable it whenever possible?


> +
>  When using other types of backends, it's suggested to set 'unarmed'
>  option of '-device nvdimm' to 'on', which sets the unarmed flag of the
>  guest NVDIMM region mapping structure.  This unarmed flag indicates
[...]
> diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
> index a9d5e56..33a7639 100644
> --- a/util/mmap-alloc.c
> +++ b/util/mmap-alloc.c
> @@ -99,7 +99,7 @@ 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;
> +    bool is_pmemsync = (flags & RAM_PMEM) && (flags & RAM_SYNC);

You seem to be reverting what you did on patch 3/5.  In patch
3/5, you were setting MAP_SYNC automatically on all pmem=on
backends.  Now, you are only setting MAP_SYNC only if sync=on is
set explicitly.

I don't know which behavior is better (see question above), but
it's better to start with the right behavior in the first place.

Also, I don't think we should clear MAP_SYNC silently if sync=on
was explicitly requested in the command-line.  If sync=on was
set, we should do exactly as told, and require MAP_SYNC.  If we
still want to support use cases where MAP_SYNC is desired but
optional (do we?), we can make 'sync' a OnOffAuto option.


>      int mmap_xflags = 0;
>      size_t offset;
>      void *ptr1;
> @@ -111,7 +111,7 @@ 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 (shared && is_pmem) {
> +    if (shared && is_pmemsync) {
>          mmap_xflags |= MAP_SYNC;
>      }
>  
> -- 
> 2.7.4
> 
> 

-- 
Eduardo