Re: [Qemu-devel] [PATCH v11 2/3] util/mmap-alloc: support MAP_SYNC in qemu_ram_mmap()

[Yi Zhang]

On 2019-01-29 at 01:55:06 -0500, Pankaj Gupta wrote:
> 
> > 
> > From: Zhang Yi <address@hidden>
> > 
> > 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).
> > 
> > Current, We have below different possible use cases:
> > 
> > 1. pmem=on is set, shared=on is set, MAP_SYNC supported:
> >    a: backend is a dax supporting file.
> >     - MAP_SYNC will active.
> >    b: backend is not a dax supporting file.
> >     - mmap will trigger a warning. then MAP_SYNC flag will be ignored
> > 
> > 2. The rest of cases:
> >    - we will never pass the MAP_SYNC to mmap2
> > 
> > Signed-off-by: Haozhong Zhang <address@hidden>
> > Signed-off-by: Zhang Yi <address@hidden>
> > ---
> >  include/qemu/osdep.h | 21 +++++++++++++++++++++
> >  util/mmap-alloc.c    | 28 +++++++++++++++++++++++++++-
> >  2 files changed, 48 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> > index 457d24e..96209bb 100644
> > --- a/include/qemu/osdep.h
> > +++ b/include/qemu/osdep.h
> > @@ -419,6 +419,27 @@ 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 <linux/mman.h>
> > +
> > +#ifndef MAP_SYNC
> > +#define MAP_SYNC 0x80000
> > +#endif
> > +
> > +#ifndef MAP_SHARED_VALIDATE
> > +#define MAP_SHARED_VALIDATE 0x03
> > +#endif
> > +
> > +#else  /* !CONFIG_LINUX */
> > +#define MAP_SYNC              0x0
> > +#define MAP_SHARED_VALIDATE   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 97bbeed..2c86ad2 100644
> > --- a/util/mmap-alloc.c
> > +++ b/util/mmap-alloc.c
> > @@ -101,6 +101,7 @@ void *qemu_ram_mmap(int fd,
> >  #else
> >      void *ptr = mmap(0, total, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1,
> >      0);
> >  #endif
> > +    int mmap_xflags = 0;
> >      size_t offset;
> >      void *ptr1;
> >  
> > @@ -111,13 +112,38 @@ void *qemu_ram_mmap(int fd,
> >      assert(is_power_of_2(align));
> >      /* Always align to host page size */
> >      assert(align >= getpagesize());
> > +    if (shared && is_pmem) {
> > +        mmap_xflags = MAP_SYNC | MAP_SHARED_VALIDATE;
> > +    }
> >  
> >      offset = QEMU_ALIGN_UP((uintptr_t)ptr, align) - (uintptr_t)ptr;
> > +retry_mmap:
> >      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 map failed with MAP_SHARED_VALIDATE | MAP_SYNC,
> > +     * we try with MAP_SHARED_VALIDATE without MAP_SYNC
> > +     */
> > +    if (ptr1 == MAP_FAILED &&
> > +        mmap_xflags == (MAP_SYNC | MAP_SHARED_VALIDATE)) {
> > +        if (errno == ENOTSUP) {
> > +            perror("failed to validate with mapping flags");
> > +        }
> > +        mmap_xflags = MAP_SHARED_VALIDATE;
> > +        goto retry_mmap;
> > +    }
> > +    /* MAP_SHARED_VALIDATE flag is available since Linux 4.15
> > +     * Test only with MAP_SHARED_VALIDATE flag for compatibility.
> > +     * Then ignore the MAP_SHARED_VALIDATE flag and retry again
> > +     */
> > +    if (mmap_xflags == MAP_SHARED_VALIDATE &&
> > +        ptr1 == MAP_FAILED) {
> > +            mmap_xflags &= ~MAP_SHARED_VALIDATE;
> > +            goto retry_mmap;
> > +    }
> 
> I am not sure if we need this multiple validation. If MAP_SYNC with 
> MAP_SHARED_VALIDATE is not supported or failed, just fallback to 
> mmap without MAP_SYNC & MAP_SHARED_VALIDATE?
Right, that is, I will improve that. Thanks Pankaj.
> 
> I saw a'lot of discussion in previous version of this patch series. 
> I am not sure if its suggested this way or I am missing anything
> important here.
> 
> Thanks,
> Pankaj
> 
> 
> >      if (ptr1 == MAP_FAILED) {
> >          munmap(ptr, total);
> >          return MAP_FAILED;
> > --
> > 2.7.4
> > 
> > 
> >