Re: [Qemu-devel] [PATCH V6 0/6] nvdimm: support MAP_SYNC for memory-backend-file

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

On Wed, Dec 12, 2018 at 04:11:44PM +0800, Zhang Yi wrote:
> Linux 4.15 introduces a new mmap flag MAP_SYNC, which can be used to
> guarantee the write persistence to mmap'ed files supporting DAX (e.g.,
> files on ext4/xfs file system mounted with '-o dax').
> 
> A description of MAP_SYNC and MAP_SHARED_VALIDATE can be found at
>     https://patchwork.kernel.org/patch/10028151/
> 
> In order to make sure that the file metadata is in sync after a fault 
> while we are writing a shared DAX supporting backend files, this
> patch-set enables QEMU to use MAP_SYNC flag for memory-backend-dax-file.
> 
> As the DAX vs DMA truncated issue was solved, we refined the code and
> send out this feature for the v5 version.
> 
> A new auto on/off option 'sync' is added to memory-backend-file:
>  - on:  try to pass MAP_SYNC to mmap(2); if MAP_SYNC is not supported or
>         'share=off', QEMU will abort
>  - off: never pass MAP_SYNC to mmap(2)
>  - auto (default): if MAP_SYNC is supported and 'share=on', work as if
>         'sync=on'; otherwise, work as if 'sync=off'

Can this change be limited to a real DAX device?
Then you won't need to bother with flags at all.

And am I mistaken in thinking that this will affect all
guest memory with share=on? Or do I misunderstand?



> Changes in v6:
>  * Pankaj: 3/7 are squashed with 2/7
>  * Pankaj: 7/7 update comments to "consistent filesystem metadata".
>  * Pankaj, Igor: 1/7 Added Reviewed-by in patch-1/7
>  * Stefan, 4/7 move the include header from "/linux/mman.h" to "osdep.h"
>  * Stefan, 5/7 Add missing "munmap"
>  * Stefan, 2/7 refine the shared/flag.
> 
> Changes in v5:
>  * Add patch 1 to fix a memory leak issue.
>  * Refine the patch 4-6
>  * Remove the patch 3 as we already change the parameter from "shared" to
>    "flags"
> 
> Changes in v4:
>  * Add patch 1-3 to switch some functions to a single 'flags'
>    parameters. (Michael S. Tsirkin)
>  * v3 patch 1-3 become v4 patch 4-6.
>  * Patch 4: move definitions of MAP_SYNC and MAP_SHARED_VALIDATE to a
>    new header file under include/standard-headers/linux/. (Michael S. Tsirkin)
>  * Patch 6: refine the description of the 'sync' option. (Michael S. Tsirkin)
> 
> Changes in v3:
>  * Patch 1: add MAP_SHARED_VALIDATE in both sync=on and sync=auto
>    cases, and add back the retry mechanism. MAP_SYNC will be ignored
>    by Linux kernel 4.15 if MAP_SHARED_VALIDATE is missed.
>  * Patch 1: define MAP_SYNC and MAP_SHARED_VALIDATE as 0 on non-Linux
>    platforms in order to make qemu_ram_mmap() compile on those platforms.
>  * Patch 2&3: include more information in error messages of
>    memory-backend in hope to help user to identify the error.
>    (Dr. David Alan Gilbert)
>  * Patch 3: fix typo in the commit message. (Dr. David Alan Gilbert)
> 
> Changes in v2:
>  * Add 'sync' option to control the use of MAP_SYNC. (Eduardo Habkost)
>  * Remove the unnecessary set of MAP_SHARED_VALIDATE in some cases and
>    the retry mechanism in qemu_ram_mmap(). (Michael S. Tsirkin)
>  * Move OS dependent definitions of MAP_SYNC and MAP_SHARED_VALIDATE
>    to osdep.h. (Michael S. Tsirkin)
> 
> Zhang Yi (6):
>   numa: Fixed the memory leak of numa error message
>   util/mmap-alloc: switch qemu_ram_mmap() to 'flags' parameter
>   util/mmap-alloc: support MAP_SYNC in qemu_ram_mmap()
>   util/mmap-alloc: Switch the RAM_SYNC flags to OnOffAuto
>   hostmem: add more information in error messages
>   hostmem-file: add 'sync' option
> 
>  backends/hostmem-file.c   | 45 +++++++++++++++++++++++++++++++++++++++++++--
>  backends/hostmem.c        |  8 +++++---
>  docs/nvdimm.txt           | 20 +++++++++++++++++++-
>  exec.c                    |  9 +++++----
>  include/exec/memory.h     | 18 ++++++++++++++++++
>  include/exec/ram_addr.h   |  1 +
>  include/qemu/mmap-alloc.h | 20 +++++++++++++++++++-
>  include/qemu/osdep.h      | 29 +++++++++++++++++++++++++++++
>  numa.c                    |  1 +
>  qemu-options.hx           | 22 +++++++++++++++++++++-
>  util/mmap-alloc.c         | 26 +++++++++++++++++++++-----
>  util/oslib-posix.c        |  8 +++++++-
>  12 files changed, 189 insertions(+), 18 deletions(-)
> 
> -- 
> 2.7.4