[V13,5/5] docs: Added MAP_SYNC documentation

Commit Message

Zhang, Yi Feb. 8, 2019, 10:11 a.m. UTC

From: Zhang Yi <[e-mail]>

Signed-off-by: Zhang Yi <[e-mail]>
---
 docs/nvdimm.txt | 25 ++++++++++++++++++++++---
 qemu-options.hx |  5 +++++
 2 files changed, 27 insertions(+), 3 deletions(-)

Comments

Michael S. Tsirkin Feb. 8, 2019, 3 a.m. UTC | #1

On Fri, Feb 08, 2019 at 06:11:23PM +0800, Zhang, Yi wrote:
> From: Zhang Yi <[e-mail]>
> 
> Signed-off-by: Zhang Yi <[e-mail]>
> ---
>  docs/nvdimm.txt | 25 ++++++++++++++++++++++---
>  qemu-options.hx |  5 +++++
>  2 files changed, 27 insertions(+), 3 deletions(-)
> 
> diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
> index 5f158a6..a168429 100644
> --- a/docs/nvdimm.txt
> +++ b/docs/nvdimm.txt
> @@ -143,9 +143,28 @@ Guest Data Persistence
>  ----------------------
>  
>  Though QEMU supports multiple types of vNVDIMM backends on Linux,
> -currently the only one 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.
> +the only backend that can guarantee the guest write persistence is:
> +
> +A. DAX device (e.g., /dev/dax0.0, ) or
> +B. DAX file(mounted with dax option)
> +
> +both are use real NVDIMM device as backend, which supporting direct
> +access for files(no page cache).

So I would just drop above two lines.

How does one know that there's support for direct access?
it's just a question of kernel right?

And I don't think it matter wrt "real NVDIMM" - e.g. it
could be vNVDIMM in a nested virt scenario.
It will still survive host (i.e. L1 guest) crash.

Besides, the above two lines aren't grammatical.

> +
> +When using B (A file supporting direct mapping of persistent memory)
> +as a backend, write persistence is guaranteed if the host kernel has
> +support for the MAP_SYNC flag in the mmap system call (available
> +since Linux 4.15 and on certain distro kernels) and additionally
> +both 'pmem' and 'share' flags are set to 'on' on the backend.
> +
> +If these conditions are not satisfied i.e. if either 'pmem' or 'share'
> +are not set, if the backend file does not support DAX or if MAP_SYNC
> +is not supported by the host kernel, write persistence is not
> +guaranteed after a system crash. For compatibility reasons, these
> +conditions are silently ignored if not satisfied. Currently, no way
> +is provided to test for them.
> +For more details, please reference mmap(2) man page:
> +http://man7.org/linux/man-pages/man2/mmap.2.html.
>  
>  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
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 08f8516..ef1da8f 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -4002,6 +4002,11 @@ using the SNIA NVM programming model (e.g. Intel NVDIMM).
>  If @option{pmem} is set to 'on', QEMU will take necessary operations to
>  guarantee the persistence of its own writes to @option{mem-path}
>  (e.g. in vNVDIMM label emulation and live migration).
> +Also, we will map the backend-file with MAP_SYNC flag, which ensures the
> +file metadata is in sync for @option{mem-path} in case of host crash
> +or a power failure. MAP_SYNC requires support from both the host kernel
> +(since Linux kernel 4.15) and the filesystem of @option{mem-path} mounted
> +with DAX option.
>  
>  @item -object memory-backend-ram,id=@var{id},merge=@var{on|off},dump=@var{on|off},share=@var{on|off},prealloc=@var{on|off},size=@var{size},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave}
>  
> -- 
> 2.7.4

Zhang, Yi Feb. 8, 2019, 1:07 p.m. UTC | #2

On 2019-02-07 at 22:00:46 -0500, Michael S. Tsirkin wrote:
> On Fri, Feb 08, 2019 at 06:11:23PM +0800, Zhang, Yi wrote:
> > From: Zhang Yi <[e-mail]>
> > 
> > Signed-off-by: Zhang Yi <[e-mail]>
> > ---
> >  docs/nvdimm.txt | 25 ++++++++++++++++++++++---
> >  qemu-options.hx |  5 +++++
> >  2 files changed, 27 insertions(+), 3 deletions(-)
> > 
> > diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
> > index 5f158a6..a168429 100644
> > --- a/docs/nvdimm.txt
> > +++ b/docs/nvdimm.txt
> > @@ -143,9 +143,28 @@ Guest Data Persistence
> >  ----------------------
> >  
> >  Though QEMU supports multiple types of vNVDIMM backends on Linux,
> > -currently the only one 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.
> > +the only backend that can guarantee the guest write persistence is:
> > +
> > +A. DAX device (e.g., /dev/dax0.0, ) or
> > +B. DAX file(mounted with dax option)
> > +
> > +both are use real NVDIMM device as backend, which supporting direct
> > +access for files(no page cache).
> 
> So I would just drop above two lines.
> 
> How does one know that there's support for direct access?
> it's just a question of kernel right?
> 
> And I don't think it matter wrt "real NVDIMM" - e.g. it
> could be vNVDIMM in a nested virt scenario.
> It will still survive host (i.e. L1 guest) crash.
> 
> Besides, the above two lines aren't grammatical.
Ah.. Reasonable, will drop these two lines.
Thanks for your kindly review. Michael.
> 
> > +
> > +When using B (A file supporting direct mapping of persistent memory)
> > +as a backend, write persistence is guaranteed if the host kernel has
> > +support for the MAP_SYNC flag in the mmap system call (available
> > +since Linux 4.15 and on certain distro kernels) and additionally
> > +both 'pmem' and 'share' flags are set to 'on' on the backend.
> > +
> > +If these conditions are not satisfied i.e. if either 'pmem' or 'share'
> > +are not set, if the backend file does not support DAX or if MAP_SYNC
> > +is not supported by the host kernel, write persistence is not
> > +guaranteed after a system crash. For compatibility reasons, these
> > +conditions are silently ignored if not satisfied. Currently, no way
> > +is provided to test for them.
> > +For more details, please reference mmap(2) man page:
> > +http://man7.org/linux/man-pages/man2/mmap.2.html.
> >  
> >  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
> > diff --git a/qemu-options.hx b/qemu-options.hx
> > index 08f8516..ef1da8f 100644
> > --- a/qemu-options.hx
> > +++ b/qemu-options.hx
> > @@ -4002,6 +4002,11 @@ using the SNIA NVM programming model (e.g. Intel NVDIMM).
> >  If @option{pmem} is set to 'on', QEMU will take necessary operations to
> >  guarantee the persistence of its own writes to @option{mem-path}
> >  (e.g. in vNVDIMM label emulation and live migration).
> > +Also, we will map the backend-file with MAP_SYNC flag, which ensures the
> > +file metadata is in sync for @option{mem-path} in case of host crash
> > +or a power failure. MAP_SYNC requires support from both the host kernel
> > +(since Linux kernel 4.15) and the filesystem of @option{mem-path} mounted
> > +with DAX option.
> >  
> >  @item -object memory-backend-ram,id=@var{id},merge=@var{on|off},dump=@var{on|off},share=@var{on|off},prealloc=@var{on|off},size=@var{size},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave}
> >  
> > -- 
> > 2.7.4

Patch

diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
index 5f158a6..a168429 100644
--- a/docs/nvdimm.txt
+++ b/docs/nvdimm.txt
@@ -143,9 +143,28 @@  Guest Data Persistence
 ----------------------
 
 Though QEMU supports multiple types of vNVDIMM backends on Linux,
-currently the only one 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.
+the only backend that can guarantee the guest write persistence is:
+
+A. DAX device (e.g., /dev/dax0.0, ) or
+B. DAX file(mounted with dax option)
+
+both are use real NVDIMM device as backend, which supporting direct
+access for files(no page cache).
+
+When using B (A file supporting direct mapping of persistent memory)
+as a backend, write persistence is guaranteed if the host kernel has
+support for the MAP_SYNC flag in the mmap system call (available
+since Linux 4.15 and on certain distro kernels) and additionally
+both 'pmem' and 'share' flags are set to 'on' on the backend.
+
+If these conditions are not satisfied i.e. if either 'pmem' or 'share'
+are not set, if the backend file does not support DAX or if MAP_SYNC
+is not supported by the host kernel, write persistence is not
+guaranteed after a system crash. For compatibility reasons, these
+conditions are silently ignored if not satisfied. Currently, no way
+is provided to test for them.
+For more details, please reference mmap(2) man page:
+http://man7.org/linux/man-pages/man2/mmap.2.html.
 
 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
diff --git a/qemu-options.hx b/qemu-options.hx
index 08f8516..ef1da8f 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -4002,6 +4002,11 @@  using the SNIA NVM programming model (e.g. Intel NVDIMM).
 If @option{pmem} is set to 'on', QEMU will take necessary operations to
 guarantee the persistence of its own writes to @option{mem-path}
 (e.g. in vNVDIMM label emulation and live migration).
+Also, we will map the backend-file with MAP_SYNC flag, which ensures the
+file metadata is in sync for @option{mem-path} in case of host crash
+or a power failure. MAP_SYNC requires support from both the host kernel
+(since Linux kernel 4.15) and the filesystem of @option{mem-path} mounted
+with DAX option.
 
 @item -object memory-backend-ram,id=@var{id},merge=@var{on|off},dump=@var{on|off},share=@var{on|off},prealloc=@var{on|off},size=@var{size},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave}