Re: [Qemu-devel] [PATCH v14 0/2] support MAP_SYNC for memory-backend-file

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

On Mon, Apr 22, 2019 at 08:48:47AM +0800, Wei Yang 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.
> 
> We will pass MAP_SYNC to mmap(2); if MAP_SYNC is supported and
> 'share=on' & 'pmem=on'. 
> Or QEMU will not pass this flag to mmap(2)

OK this is in a good shape. As we are in freeze anyway,
there's still a bit more time to polish it. I have a couple of
suggestions:

- squash docs in same patch with code, no need for two patches
- mmap errors are not silently ignored as the doc says,
  a warning is produced

Also, it might make sense to send the warnings to an errp object and not stderr.
I would leave that to a follow-up patch.


> Test with below cases:
> 1. pmem=on is set, shared=on is set, MAP_SYNC supported:
>    a: backend is a dax supporting file.
>    1) start VM1 with options:
>    -object memory-backend-file,id=nv_be4,share,mem-path=${DAX_FILE_1},size=${DAX_FILE_SIZE_1},align=128M,pmem=on,share=on
>    -device nvdimm,id=nv4,memdev=nv_be4,label-size=2M.
>    
>    2) start VM2 with options:
>    -object memory-backend-file,id=nv_be4,share,mem-path=${DAX_FILE_2,size=${DAX_FILE_SIZE_2},align=128M,pmem=on,share=on
>    -device nvdimm,id=nv4,memdev=nv_be4,label-size=2M.
> 
>    3) live migrate from VM1 to VM2.
>    
>    4) Suddenly let Host crash or power failure.
> 
>    5) check DAX_FILE_1 and DAX_FILE_2, no corrupt.
> 
>    b: backend is a regular file.
>    1) start with options
>    -object memory-backend-file,id=nv_be4,share,mem-path=${REG_FILE},size=${REG_FILE_SIZE},align=128M,pmem=on,share=on
>    -device nvdimm,id=nv4,memdev=nv_be4,label-size=2M.
> 
>    will warning "failed to validate with mapping flags: Operation not supported"
>    FILE_1 and FILE_2 random corrupt.
> 
> 2. Other cases:
>    FILE_1 and FILE_2 random corrupt.
> 
> Changes in V14:
>  * 1/2 rebase on top of current upstream and tested
> 
> Changes in V13:
>  * 4/5 Micheal: move the inlcude to mmap_alloc.c.
>  * 4/5 Micheal: refine the warning message.
>  * 5/5 Micheal: refine the Documentations.
> 
> Changes in V12:
>  * 2/5: Micheal: Update update-linux-headers.sh
>  * 3/5: Micheal: Use script update add linux/mman.h
>  * 4/5: Pankaj,Micheal: 1) fallback to mmap without
>         MAP_SYNC & MAP_SHARED_VALIDATE if sync not supported or failed
> 	2) Replace the include with 3/5 added linux/mman.h
>  * 5/5: Micheal: Refine the Documentations.
> 
> Changes in V11:
>  * 1/3: Micheal: Change to just add a bool is_pmem in qemu_ram_mmap.
>  * 2/3: Micheal: Fix the compatibility for old kernel.
>  * 2/3&3/3: Micheal&Eduardo :Update the behavior below: 
>    Waning at no-dax and continue without MAP_SYNC.
>    Test if fails again for compatibility, then remove the MAP_VALIDATE and
>    silently proceed.
> 
> Changes in V10:
>  * 4/4: refine the document.
>  * 3/4: Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
>  * 2/4: refine the commit message, Added MAP_SHARED_VALIDATE.
>  * 2/4: Fix the wrong include header
> 
> Changes in V9:
>  * 1/6: Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
>  * 2/6: New Added: Micheal: use sparse feature define RAM_FLAG. 
>  since I don't have much knowledge about the sparse feature, @Micheal Could you 
>  add some documentation/commit message on this patch? Thank you very much.
>  * 3/6: from 2/5: Eduardo: updated the commit message. 
>  * 4/6: from 3/5: Micheal: don't ignore MAP_SYNC failures silently.
>  * 5/6: from 4/5: Eduardo: updated the commit message.
>  * 6/6: from 5/5: Micheal: Drop the sync option, document the MAP_SYNC.
> 
> Changes in v8:
>  * Micheal: 3/5, remove the duplicated define in the os_dep.h
>  * Micheal: 2/5, make type define safety.
>  * Micheal: 2/5, fixed the incorrect define MAP_SHARE on qemu_anon_ram_alloc.
>  * 4/6 removed, we remove the on/off/auto define of sync,  as by now,
>    MAP_SYNC only worked with pmem=on.
>  * @Micheal, I still reuse the RAM_SYNC flag, it is much straightforward to parse 
>    all the flags in one parameter.
> 
> Changes in v7:
>  * Micheal: [3,4,6]/6 limited the "sync" flag only on a nvdimm backend.(pmem=on)
> 
> 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 (2):
>   util/mmap-alloc: support MAP_SYNC in qemu_ram_mmap()
>   docs: Added MAP_SYNC documentation
> 
>  docs/nvdimm.txt   | 22 +++++++++++++++++++---
>  qemu-options.hx   |  5 +++++
>  util/mmap-alloc.c | 41 ++++++++++++++++++++++++++++++++++++++++-
>  3 files changed, 64 insertions(+), 4 deletions(-)
> 
> -- 
> 2.19.1