Re: [Qemu-devel] [PATCH V8 3/5] util/mmap-alloc: support MAP_SYNC in qemu_ram_mmap()

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

On Tue, Jan 15, 2019 at 10:49:45AM +0800, Yi Zhang wrote:
> On 2019-01-14 at 17:07:02 -0200, Eduardo Habkost wrote:
> > On Wed, Jan 02, 2019 at 01:26:15PM +0800, Zhang Yi wrote:
> > > When a file supporting DAX is used as vNVDIMM backend, mmap it with
> > > MAP_SYNC flag in addition which can ensure file system metadata
> > > synced in each guest writes to the backend file, without other QEMU
> > > actions (e.g., periodic fsync() by QEMU).
> > > 
> > > Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> > > Signed-off-by: Zhang Yi <yi.z.zhang@linux.intel.com>
> > > ---
> > >  include/qemu/osdep.h | 16 ++++++++++++++++
> > >  util/mmap-alloc.c    | 12 +++++++++++-
> > >  2 files changed, 27 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> > > index 3bf48bc..bb1eba1 100644
> > > --- a/include/qemu/osdep.h
> > > +++ b/include/qemu/osdep.h
> > > @@ -410,6 +410,22 @@ void qemu_anon_ram_free(void *ptr, size_t size);
> > >  #  define QEMU_VMALLOC_ALIGN getpagesize()
> > >  #endif
> > >  
> > > +/*
> > > + * MAP_SHARED_VALIDATE and MAP_SYNC are introduced in Linux kernel
> > > + * 4.15, so they may not be defined when compiling on older kernels.
> > > + */
> > > +#ifdef CONFIG_LINUX
> > > +
> > > +#include <asm-generic/mman.h>
> > > +
> > > +#ifndef MAP_SYNC
> > > +#define MAP_SYNC 0x0
> > > +#endif
> > > +
> > > +#else  /* !CONFIG_LINUX */
> > > +#define MAP_SYNC              0x0
> > > +#endif /* CONFIG_LINUX */
> > > +
> > >  #ifdef CONFIG_POSIX
> > >  struct qemu_signalfd_siginfo {
> > >      uint32_t ssi_signo;   /* Signal number */
> > > diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
> > > index 8f0a740..a9d5e56 100644
> > > --- a/util/mmap-alloc.c
> > > +++ b/util/mmap-alloc.c
> > > @@ -99,6 +99,8 @@ 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;
> > > +    int mmap_xflags = 0;
> > >      size_t offset;
> > >      void *ptr1;
> > >  
> > > @@ -109,13 +111,21 @@ 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) {
> > > +        mmap_xflags |= MAP_SYNC;
> > > +    }
> > >  
> > >      offset = QEMU_ALIGN_UP((uintptr_t)ptr, align) - (uintptr_t)ptr;
> > > + retry_mmap_fd:
> > >      ptr1 = mmap(ptr + offset, size, PROT_READ | PROT_WRITE,
> > >                  MAP_FIXED |
> > >                  (fd == -1 ? MAP_ANONYMOUS : 0) |
> > > -                (shared ? MAP_SHARED : MAP_PRIVATE),
> > > +                (shared ? MAP_SHARED : MAP_PRIVATE) | mmap_xflags,
> > >                  fd, 0);
> > > +    if ((ptr1 == MAP_FAILED) && (mmap_xflags & MAP_SYNC)) {
> > > +        mmap_xflags &= ~MAP_SYNC;
> > > +        goto retry_mmap_fd;
> > 
> > Do we have use cases where using pmem=on without MAP_SYNC isn't
> > going to cause problems?  If not, shouldn't we at least print a
> Yes, we have a case that direct use dax device but not a files on
> dax aware file system, we prefer to don't set the MAP_SYNC if user
> haven't much knowledge about that. it may took some potencial 
> performance issues with MAP_SYNC.

I think you will have to be quite a bit more specific.

If there's a performance / functionality tradeoff here
then hiding it behind an option with an inscrutable name
isn't a good idea. Neither is ignoring failures silently.



> > warning here?  Otherwise, won't we still need an option for cases
> > that require MAP_SYNC to be working?
> > 
> > > +    }
> > 
> > -- 
> > Eduardo