Re: [Qemu-devel] [RFC 05/29] postcopy: use UFFDIO_ZEROPAGE only when available

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

* Peter Xu (peterx@redhat.com) wrote:
> On Wed, Jun 28, 2017 at 08:00:23PM +0100, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > Use the recently added migration flag to hold whether
> > each RAMBlock has the UFFDIO_ZEROPAGE capability, use it
> > when it's available.
> > 
> > This allows the use of postcopy on tmpfs as well as hugepage
> > backed files.
> > 
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > ---
> >  migration/migration.h    |  4 ++++
> >  migration/postcopy-ram.c | 12 +++++++++---
> >  2 files changed, 13 insertions(+), 3 deletions(-)
> > 
> > diff --git a/migration/migration.h b/migration/migration.h
> > index d9a268a3af..d109635d08 100644
> > --- a/migration/migration.h
> > +++ b/migration/migration.h
> > @@ -20,6 +20,10 @@
> >  #include "exec/cpu-common.h"
> >  #include "qemu/coroutine_int.h"
> >  
> > +/* Migration flags to be set using qemu_ram_set_migration_flags */
> > +/* Postcopy can atomically zero pages in this RAMBlock */
> > +#define QEMU_MIGFLAG_POSTCOPY_ZERO   0x00000001
> > +
> >  /* State for the incoming migration */
> >  struct MigrationIncomingState {
> >      QEMUFile *from_src_file;
> > diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> > index be2a8f8e02..96338a8070 100644
> > --- a/migration/postcopy-ram.c
> > +++ b/migration/postcopy-ram.c
> > @@ -408,6 +408,12 @@ static int ram_block_enable_notify(const char *block_name, void *host_addr,
> >          error_report("%s userfault: Region doesn't support COPY", __func__);
> >          return -1;
> >      }
> > +    if (reg_struct.ioctls & ((__u64)1 << _UFFDIO_ZEROPAGE)) {
> > +        RAMBlock *rb = qemu_ram_block_by_name(block_name);
> > +        qemu_ram_set_migration_flags(rb, qemu_ram_get_migration_flags(rb) |
> > +                                         QEMU_MIGFLAG_POSTCOPY_ZERO);
> 
> Shall we use atomic_or() inside qemu_ram_set_migration_flags()? Then
> no need to fetch, and we'll be thread safe as well?

I've changed it to a simple |=  in the new qemu_ram_set_uf_zeroable
that works on the rb->flags field.

> > +    }
> > +
> >  
> >      return 0;
> >  }
> > @@ -620,11 +626,11 @@ int postcopy_place_page(MigrationIncomingState *mis, void *host, void *from,
> >  int postcopy_place_page_zero(MigrationIncomingState *mis, void *host,
> >                               RAMBlock *rb)
> >  {
> > +    size_t pagesize = qemu_ram_pagesize(rb);
> >      trace_postcopy_place_page_zero(host);
> >  
> > -    if (qemu_ram_pagesize(rb) == getpagesize()) {
> > -        if (qemu_ufd_copy_ioctl(mis->userfault_fd, host, NULL, getpagesize(),
> > -                                rb)) {
> > +    if (qemu_ram_get_migration_flags(rb) & QEMU_MIGFLAG_POSTCOPY_ZERO) {
> 
> IIUC, _UFFDIO_ZEROPAGE is not supported on huge pages. If so, would
> here worth a comment?

Added.

Dave

> > +        if (qemu_ufd_copy_ioctl(mis->userfault_fd, host, NULL, pagesize, rb)) {
> >              int e = errno;
> >              error_report("%s: %s zero host: %p",
> >                           __func__, strerror(e), host);
> > -- 
> > 2.13.0
> > 
> 
> Thanks,
> 
> -- 
> Peter Xu
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK