Re: [Qemu-devel] [PATCH V7 3/6] util/mmap-alloc: support MAP_SYNC in qemu_ram_mmap()

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

On Wed, Dec 19, 2018 at 05:25:29PM +0800, Yi Zhang wrote:
> On 2018-12-18 at 08:52:09 -0500, Michael S. Tsirkin wrote:
> > On Tue, Dec 18, 2018 at 04:17:12PM +0800, Zhang Yi wrote:
> > > When a file supporting DAX is used as vNVDIMM backend, mmap it with
> > > MAP_SYNC flag in addition can guarantee the persistence of guest write
> > > to the backend file without other QEMU actions (e.g., periodic fsync()
> > > by QEMU).
> > > 
> > > A set of RAM_SYNC flags are added to qemu_ram_mmap():
> > > 
> > > Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> > > Signed-off-by: Zhang Yi <yi.z.zhang@linux.intel.com>
> > 
> > Should we be reusing RAM_ flags? These are used widely
> > already -are you going to allow RAM_SYNC everywhere
> > where other RAM flags are legal?
> > E.g. memory_region_init_ram_from_file?
> To reuse the RAM_* is much straightforward, just like RAM_SHARE, it is a
> mmmap flag, and we will check the inlegal flags at a easy way.
> 
> assert((ram_flags & ~(RAM_SHARED | RAM_PMEM | RAM_SYNC)) == 0);

Doesn't help if a flag misuse happens to match one of these.

> What do you think, do you have a better choice? Thanks, Micheal.

How about your own flag and static checking with e.g. sparse?

> > 
> > 
> > > ---
> > >  exec.c                    |  2 +-
> > >  include/exec/memory.h     |  3 +++
> > >  include/exec/ram_addr.h   |  1 +
> > >  include/qemu/mmap-alloc.h |  1 +
> > >  include/qemu/osdep.h      | 29 +++++++++++++++++++++++++++++
> > >  util/mmap-alloc.c         | 14 ++++++++++----
> > >  6 files changed, 45 insertions(+), 5 deletions(-)
> > > 
> > > 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 667466b..33a4e2c 100644
> > > --- a/include/exec/memory.h
> > > +++ b/include/exec/memory.h
> > > @@ -126,6 +126,9 @@ typedef struct IOMMUNotifier IOMMUNotifier;
> > >  /* RAM is a persistent kind memory */
> > >  #define RAM_PMEM (1 << 5)
> > >  
> > > +/* RAM can be mmap by a MAP_SYNC flag */
> > 
> > "RAM is mmap-ed with MAP_SYNC"
> > 
> > > +#define RAM_SYNC (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/include/qemu/osdep.h b/include/qemu/osdep.h
> > > index 3bf48bc..f94ea68 100644
> > > --- a/include/qemu/osdep.h
> > > +++ b/include/qemu/osdep.h
> > > @@ -410,6 +410,35 @@ 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.
> > 
> > Then you want to import the relevant headers into linux-headers
> > using scripts/update-linux-headers.sh. Pls do not duplicate code.
> Indeed, will update it. 
> > 
> > > + */
> > > +#ifdef CONFIG_LINUX
> > > +
> > > +#include <sys/mman.h>
> > > +
> > > +#ifndef MAP_SHARED_VALIDATE
> > > +#define MAP_SHARED_VALIDATE   0x3
> > > +#endif
> > > +
> > > +#ifndef MAP_SYNC
> > > +#define MAP_SYNC              0x80000
> > > +#endif
> > > +
> > > +/* MAP_SYNC is only available with MAP_SHARED_VALIDATE. */
> > > +#define MAP_SYNC_FLAGS (MAP_SYNC | MAP_SHARED_VALIDATE)
> > 
> > Given mmap flags tend to start with MAP_, adding our own ones
> > unconditionally is not a good idea.
> How about to move the define to other place? 

I'd just open-code, it's not used so widely.

> > 
> > 
> > > +
> > > +#else  /* !CONFIG_LINUX */
> > > +
> > > +#define MAP_SHARED_VALIDATE   0x0
> > > +#define MAP_SYNC              0x0
> > > +
> > > +#define QEMU_HAS_MAP_SYNC     false
> > 
> > Isn't just checking MAP_SYNC enough?
> > 
> > > +#define MAP_SYNC_FLAGS 0
> > > +
> > > +#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..89ae862 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;
> > >  
> > > @@ -109,16 +111,20 @@ 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 ((flags & RAM_SYNC) && shared && is_pmem) {
> > > +        mmap_xflags |= MAP_SYNC_FLAGS;
> > > +    }
> > >  
> > >      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) {
> > > -        munmap(ptr, total);
> > > -        return MAP_FAILED;
> > > +    if ((ptr1 == MAP_FAILED) && (mmap_xflags & MAP_SYNC_FLAGS)) {
> > > +        mmap_xflags &= ~MAP_SYNC_FLAGS;
> > > +        goto retry_mmap_fd;
> > >      }
> > >  
> > >      if (offset > 0) {
> > > -- 
> > > 2.7.4