Re: [Qemu-devel] [RFC v2 03/32] migrate: Update ram_block_discard_range for shared

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Xu (peterx@redhat.com) wrote:
> On Thu, Aug 24, 2017 at 08:27:01PM +0100, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > The choice of call to discard a block is getting more complicated
> > for other cases.   We use fallocate PUNCH_HOLE in any file cases;
> > it works for both hugepage and for tmpfs.
> > We use the DONTNEED for non-hugepage cases either where they're
> > anonymous or where they're private.
> > 
> > Care should be taken when trying other backing files.
> > 
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > ---
> >  exec.c       | 35 ++++++++++++++++++++++++-----------
> >  trace-events |  3 +++
> >  2 files changed, 27 insertions(+), 11 deletions(-)
> > 
> > diff --git a/exec.c b/exec.c
> > index d20c34ca83..67df2909ce 100644
> > --- a/exec.c
> > +++ b/exec.c
> > @@ -3573,6 +3573,7 @@ int ram_block_discard_range(RAMBlock *rb, uint64_t start, size_t length)
> >      }
> >  
> >      if ((start + length) <= rb->used_length) {
> > +        bool need_madvise, need_fallocate;
> >          uint8_t *host_endaddr = host_startaddr + length;
> >          if ((uintptr_t)host_endaddr & (rb->page_size - 1)) {
> >              error_report("ram_block_discard_range: Unaligned end address: %p",
> > @@ -3582,23 +3583,35 @@ int ram_block_discard_range(RAMBlock *rb, uint64_t start, size_t length)
> >  
> >          errno = ENOTSUP; /* If we are missing MADVISE etc */
> >  
> > -        if (rb->page_size == qemu_host_page_size) {
> > -#if defined(CONFIG_MADVISE)
> > -            /* Note: We need the madvise MADV_DONTNEED behaviour of definitely
> > -             * freeing the page.
> > -             */
> > -            ret = madvise(host_startaddr, length, MADV_DONTNEED);
> > -#endif
> > -        } else {
> > -            /* Huge page case  - unfortunately it can't do DONTNEED, but
> > -             * it can do the equivalent by FALLOC_FL_PUNCH_HOLE in the
> > -             * huge page file.
> > +        /* The logic here is messy;
> > +         *    madvise DONTNEED fails for hugepages
> > +         *    fallocate works on hugepages and shmem
> > +         */
> > +        need_madvise = (rb->page_size == qemu_host_page_size);
> > +        need_fallocate = rb->fd != -1;
> > +        if (need_fallocate) {
> > +            /* For a file, this causes the area of the file to be zero'd
> > +             * if read, and for hugetlbfs also causes it to be unmapped
> > +             * so a userfault will trigger.
> >               */
> >  #ifdef CONFIG_FALLOCATE_PUNCH_HOLE
> >              ret = fallocate(rb->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
> >                              start, length);
> >  #endif
> >          }
> > +        /* i.e. need madvise but skip it if the fallocate failed */
> > +        if (need_madvise && (!need_fallocate || (ret == 0))) {
> 
> I'll slightly prefer:
> 
>   trace_ram_block_discard_range();
> 
>   if (need_fallocate) {
>     ret = fallocate();
>     if (ret) {
>       error_report();
>       goto err;
>     }
>   }
> 
>   if (need_madvise) {
>     ret = madvise();
>     if (ret) {
>       error_report();
>       goto err;
>     }
>   }

OK, I've reworked it more like that.
(It's a little more complex because of the ifdef's)

Dave

> But it is personal preference.  For either way:
> 
> Reviewed-by: Peter Xu <peterx@redhat.com>
> 
> > +            /* For normal RAM this causes it to be unmapped,
> > +             * for shared memory it causes the local mapping to disappear
> > +             * and to fall back on the file contents (which we just
> > +             * fallocate'd away).
> > +             */
> > +#if defined(CONFIG_MADVISE)
> > +            ret =  madvise(host_startaddr, length, MADV_DONTNEED);
> > +#endif
> > +        }
> > +        trace_ram_block_discard_range(rb->idstr, host_startaddr,
> > +                                      need_madvise, need_fallocate, ret);
> >          if (ret) {
> >              ret = -errno;
> >              error_report("ram_block_discard_range: Failed to discard range "
> > diff --git a/trace-events b/trace-events
> > index 1f50f56d9d..213ee34f89 100644
> > --- a/trace-events
> > +++ b/trace-events
> > @@ -55,6 +55,9 @@ dma_complete(void *dbs, int ret, void *cb) "dbs=%p ret=%d cb=%p"
> >  dma_blk_cb(void *dbs, int ret) "dbs=%p ret=%d"
> >  dma_map_wait(void *dbs) "dbs=%p"
> >  
> > +# exec.c
> > +ram_block_discard_range(const char *rbname, void *hva, bool need_madvise, bool need_fallocate, int ret) "%s@%p: madvise: %d fallocate: %d ret: %d"
> > +
> >  # memory.c
> >  memory_region_ops_read(int cpu_index, void *mr, uint64_t addr, uint64_t value, unsigned size) "cpu %d mr %p addr 0x%"PRIx64" value 0x%"PRIx64" size %u"
> >  memory_region_ops_write(int cpu_index, void *mr, uint64_t addr, uint64_t value, unsigned size) "cpu %d mr %p addr 0x%"PRIx64" value 0x%"PRIx64" size %u"
> > -- 
> > 2.13.5
> > 
> 
> -- 
> Peter Xu
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK