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

[Yi Zhang]

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 <address@hidden>
> > Signed-off-by: Zhang Yi <address@hidden>
> > ---
> >  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.
> warning here?  Otherwise, won't we still need an option for cases
> that require MAP_SYNC to be working?
> 
> > +    }
> 
> -- 
> Eduardo