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

[Yi Zhang]

On 2019-01-14 at 17:39:38 -0200, Eduardo Habkost 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>
> > ---
> [...]
> > +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.
Actually, I did this on previous version.
see https://patchwork.kernel.org/patch/10725671/ 

Michael said that we should limit that option as it is only valided
on a dax aware file system, to avoid the potencial performance issues
we set it off by-defualt, and let a well-know user decides they wanna
performance or stability.
> 
> 
> >      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