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

[Yi Zhang]

On 2019-01-14 at 22:31:45 -0500, Michael S. Tsirkin wrote:
> 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>
> 
> 
> So we introduce all of the above complexity and then I am pretty sure go
> on and teach management tools to just always, without exception, set
> sync=on to avoid data corruption.
> 
> So how about we give up on a bit of flexibility, and just say
> pmem=on forces MAP_SYNC?
> 
> OTOH if you really really want a fast memory then why set pmem=on at
> all?

Indeed, All my concern is that we do need to pass the sync to a type of
pmem which didn't backend on a dax aware file. Anyway, I will drop the
sync option, and let it on while we set pmem, Thanks your suggestion.
Michael.

> 
> Or, if you have some data that shows how disabling synchronous
> pagefaults helps performance a lot, maybe we should introduce
> a "crash-unsafe" flag.
> 
> 
> 
> > ---
> >  backends/hostmem-file.c   | 28 ++++++++++++++++++++++++++++
> >  docs/nvdimm.txt           | 23 ++++++++++++++++++++++-
> >  exec.c                    |  2 +-
> >  include/exec/memory.h     |  4 ++++
> >  include/exec/ram_addr.h   |  1 +
> >  include/qemu/mmap-alloc.h |  1 +
> >  qemu-options.hx           | 19 ++++++++++++++++++-
> >  util/mmap-alloc.c         |  4 ++--
> >  8 files changed, 77 insertions(+), 5 deletions(-)
> > 
> > diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
> > index 0dd7a90..3d39032 100644
> > --- a/backends/hostmem-file.c
> > +++ b/backends/hostmem-file.c
> > @@ -36,6 +36,7 @@ struct HostMemoryBackendFile {
> >      uint64_t align;
> >      bool discard_data;
> >      bool is_pmem;
> > +    bool sync;
> >  };
> >  
> >  static void
> > @@ -62,6 +63,7 @@ file_backend_memory_alloc(HostMemoryBackend *backend, 
> > Error **errp)
> >                                   path,
> >                                   backend->size, fb->align,
> >                                   (backend->share ? RAM_SHARED : 0) |
> > +                                 (fb->sync ? RAM_SYNC : 0) |
> >                                   (fb->is_pmem ? RAM_PMEM : 0),
> >                                   fb->mem_path, errp);
> >          g_free(path);
> > @@ -136,6 +138,29 @@ static void file_memory_backend_set_align(Object *o, 
> > Visitor *v,
> >      error_propagate(errp, local_err);
> >  }
> >  
> > +static bool file_memory_backend_get_sync(Object *o, Error **errp)
> > +{
> > +    return MEMORY_BACKEND_FILE(o)->sync;
> > +}
> > +
> > +static void file_memory_backend_set_sync(
> > +    Object *obj, bool value, Error **errp)
> > +{
> > +    HostMemoryBackend *backend = MEMORY_BACKEND(obj);
> > +    HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(obj);
> > +
> > +    if (host_memory_backend_mr_inited(backend)) {
> > +        error_setg(errp, "cannot change property sync of %s",
> > +                   object_get_typename(obj));
> > +        goto out;
> > +    }
> > +
> > +    fb->sync = value;
> > +
> > + out:
> > +    return;
> > +}
> > +
> >  static bool file_memory_backend_get_pmem(Object *o, Error **errp)
> >  {
> >      return MEMORY_BACKEND_FILE(o)->is_pmem;
> > @@ -203,6 +228,9 @@ file_backend_class_init(ObjectClass *oc, void *data)
> >      object_class_property_add_bool(oc, "pmem",
> >          file_memory_backend_get_pmem, file_memory_backend_set_pmem,
> >          &error_abort);
> > +    object_class_property_add_bool(oc, "sync",
> > +        file_memory_backend_get_sync, file_memory_backend_set_sync,
> > +        &error_abort);
> >  }
> >  
> >  static void file_backend_instance_finalize(Object *o)
> > diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
> > index 5f158a6..30db458 100644
> > --- a/docs/nvdimm.txt
> > +++ b/docs/nvdimm.txt
> > @@ -142,11 +142,32 @@ backend of vNVDIMM:
> >  Guest Data Persistence
> >  ----------------------
> >  
> > +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'
> > +
> >  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/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 6e30c23..9297b1c 100644
> > --- a/include/exec/memory.h
> > +++ b/include/exec/memory.h
> > @@ -26,6 +26,7 @@
> >  #include "qom/object.h"
> >  #include "qemu/rcu.h"
> >  #include "hw/qdev-core.h"
> > +#include "qapi/error.h"
> >  
> >  #define RAM_ADDR_INVALID (~(ram_addr_t)0)
> >  
> > @@ -136,6 +137,9 @@ typedef unsigned QEMU_BITWISE QemuMmapFlags;
> >  /* RAM is a persistent kind memory */
> >  #define RAM_PMEM ((QEMU_FORCE QemuMmapFlags) (1 << 5))
> >  
> > +/* RAM can be mmap by a MAP_SYNC flag */
> > +#define RAM_SYNC ((QEMU_FORCE QemuMmapFlags) (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/qemu-options.hx b/qemu-options.hx
> > index 08f8516..0f51d08 100644
> > --- a/qemu-options.hx
> > +++ b/qemu-options.hx
> > @@ -3928,7 +3928,7 @@ property must be set.  These objects are placed in the
> >  
> >  @table @option
> >  
> > address@hidden -object 
> > memory-backend-file,address@hidden,address@hidden,address@hidden,address@hidden|off},address@hidden|off},address@hidden|off},address@hidden|off},address@hidden|off},address@hidden,address@hidden|preferred|bind|interleave},address@hidden
> > address@hidden -object 
> > memory-backend-file,address@hidden,address@hidden,address@hidden,address@hidden|off},address@hidden|off},address@hidden|off},address@hidden|off},address@hidden|off},address@hidden,address@hidden|preferred|bind|interleave},address@hidden,address@hidden|off}
> >  
> >  Creates a memory file backend object, which can be used to back
> >  the guest RAM with huge pages.
> > @@ -4003,6 +4003,23 @@ 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).
> >  
> > +The @option{sync} option specifies whether QEMU mmap(2) @option{mem-path}
> > +with MAP_SYNC flag, which can ensure the file metadata is in sync to
> > address@hidden even on the host crash and power failures. MAP_SYNC
> > +requires supports from both the host kernel (since Linux kernel 4.15)
> > +and @option{mem-path} (only files supporting DAX). It can take one of
> > +following values:
> > +
> > address@hidden @option
> > address@hidden @var{on}
> > +try to pass MAP_SYNC to mmap(2); if MAP_SYNC is not supported or
> > address@hidden@var{off}, @address@hidden QEMU will not pass
> > +this flags to kernel.
> > +
> > address@hidden @var{off} (default)
> > +never pass MAP_SYNC to mmap(2)
> > address@hidden table
> > +
> >  @item -object 
> > memory-backend-ram,address@hidden,address@hidden|off},address@hidden|off},address@hidden|off},address@hidden|off},address@hidden,address@hidden,address@hidden|preferred|bind|interleave}
> >  
> >  Creates a memory backend object, which can be used to back the guest RAM.
> > 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);
> >      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