Re: [Qemu-devel] [PATCH 3/6] migration: add UFFD_FEATURE_THREAD_ID feature support

[Alexey <a.perevalov@samsung.com>]

+ Andrea Arcangeli

On Mon, Apr 24, 2017 at 06:10:02PM +0100, Dr. David Alan Gilbert wrote:
> * Alexey (a.perevalov@samsung.com) wrote:
> > On Mon, Apr 24, 2017 at 04:12:29PM +0800, Peter Xu wrote:
> > > On Fri, Apr 21, 2017 at 06:22:12PM +0300, Alexey wrote:
> > > > On Fri, Apr 21, 2017 at 11:24:54AM +0100, Dr. David Alan Gilbert wrote:
> > > > > * Alexey Perevalov (a.perevalov@samsung.com) wrote:
> > > > > > Userfaultfd mechanism is able to provide process thread id,
> > > > > > in case when client request it with UFDD_API ioctl.
> > > > > > 
> > > > > > Signed-off-by: Alexey Perevalov <a.perevalov@samsung.com>
> > > > > 
> > > > > There seem to be two parts to this:
> > > > >   a) Adding the mis parameter to ufd_version_check
> > > > >   b) Asking for the feature
> > > > > 
> > > > > Please split it into two patches.
> > > > > 
> > > > > Also....
> > > > > 
> > > > > > ---
> > > > > >  include/migration/postcopy-ram.h |  2 +-
> > > > > >  migration/migration.c            |  2 +-
> > > > > >  migration/postcopy-ram.c         | 12 ++++++------
> > > > > >  migration/savevm.c               |  2 +-
> > > > > >  4 files changed, 9 insertions(+), 9 deletions(-)
> > > > > > 
> > > > > > diff --git a/include/migration/postcopy-ram.h b/include/migration/postcopy-ram.h
> > > > > > index 8e036b9..809f6db 100644
> > > > > > --- a/include/migration/postcopy-ram.h
> > > > > > +++ b/include/migration/postcopy-ram.h
> > > > > > @@ -14,7 +14,7 @@
> > > > > >  #define QEMU_POSTCOPY_RAM_H
> > > > > >  
> > > > > >  /* Return true if the host supports everything we need to do postcopy-ram */
> > > > > > -bool postcopy_ram_supported_by_host(void);
> > > > > > +bool postcopy_ram_supported_by_host(MigrationIncomingState *mis);
> > > > > >  
> > > > > >  /*
> > > > > >   * Make all of RAM sensitive to accesses to areas that haven't yet been written
> > > > > > diff --git a/migration/migration.c b/migration/migration.c
> > > > > > index ad4036f..79f6425 100644
> > > > > > --- a/migration/migration.c
> > > > > > +++ b/migration/migration.c
> > > > > > @@ -802,7 +802,7 @@ void qmp_migrate_set_capabilities(MigrationCapabilityStatusList *params,
> > > > > >           * special support.
> > > > > >           */
> > > > > >          if (!old_postcopy_cap && runstate_check(RUN_STATE_INMIGRATE) &&
> > > > > > -            !postcopy_ram_supported_by_host()) {
> > > > > > +            !postcopy_ram_supported_by_host(NULL)) {
> > > > > >              /* postcopy_ram_supported_by_host will have emitted a more
> > > > > >               * detailed message
> > > > > >               */
> > > > > > diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> > > > > > index dc80dbb..70f0480 100644
> > > > > > --- a/migration/postcopy-ram.c
> > > > > > +++ b/migration/postcopy-ram.c
> > > > > > @@ -60,13 +60,13 @@ struct PostcopyDiscardState {
> > > > > >  #include <sys/eventfd.h>
> > > > > >  #include <linux/userfaultfd.h>
> > > > > >  
> > > > > > -static bool ufd_version_check(int ufd)
> > > > > > +static bool ufd_version_check(int ufd, MigrationIncomingState *mis)
> > > > > >  {
> > > > > >      struct uffdio_api api_struct;
> > > > > >      uint64_t ioctl_mask;
> > > > > >  
> > > > > >      api_struct.api = UFFD_API;
> > > > > > -    api_struct.features = 0;
> > > > > > +    api_struct.features = UFFD_FEATURE_THREAD_ID;
> > > > > >      if (ioctl(ufd, UFFDIO_API, &api_struct)) {
> > > > > >          error_report("postcopy_ram_supported_by_host: UFFDIO_API failed: %s",
> > > > > >                       strerror(errno));
> > > > > 
> > > > > You're not actually using the 'mis' here - what I'd expected was
> > > > > something that was going to check if the UFFDIO_API return said that it really
> > > > > had the feature, and if so store a flag in the MIS somewhere.
> > > > > 
> > > > > Also, I'm not sure it's right to set 'api_struct.features' on the input - what
> > > > > happens if this is run on an old kernel - we don't want postcopy to fail on
> > > > > an old kernel without your feature.
> > > > > I'm not 100% sure of the interface, but I think the way it works is you set
> > > > > features = 0 before the call, and then check the api_struct.features in the
> > > > > return - in the same way that I check for UFFD_FEATURE_MISSING_HUGETLBFS.
> > > > > 
> > > > We need to ask kernel about that feature,
> > > > right,
> > > > kernel returns back available features
> > > > uffdio_api.features = UFFD_API_FEATURES
> > > > but it also stores requested features
> > > 
> > > I feel like this does not against Dave's comment, maybe we just need
> > > to send the UFFDIO_API twice? Like:
> > yes, ioctl with UFFDIO_API will fail on old kernel if we will request
> > e.g. UFFD_FEATURE_THREAD_ID or other new feature.
> > 
> > So in general way need a per feature request, for better error handling.
> 
> No, we don't need to - I think the way the kernel works is that you pass
> features = 0 in, and it sets api_struct.features on the way out;
> so if you always pass 0 in, you can then just check the features that
> it returns.
>
Without explicitly set UFFD_FEATURE_THREAD_ID, ptid will not sent back
to user space.

Also it's impossible to call ioctl UFFD_API more than one time, due to
internal state of userfault_ctx inside kernel is changing
UFFD_STATE_WAIT_API -> UFFD_STATE_RUNNING, 
but ioctl UFFD_API expects UFFD_STATE_WAIT_API
^^^

So looks like no way to provide backward compatibility for old kernels.
I even don't know how to be with new kernels, because point of extension
should be for new kernels (e.g. I want to add new feature in future,
UFFD_FEATURE_ALLOW_PADDING which will allow UFFD_COPY for lesser page
size than was registered).
So how to be in this case, add new UFFD feature, like
UFFD_FEATURE_ALLOW_CALL_API_AGAIN (allow set not always/persistent feature,
like UFFD_FEATURE_THREAD_ID)

or just remove condition in kernel while sending ptid.

Or it's even not a problem, just close ufd/reopen and resend
UFFD_FEATURE_THREAD_ID.

> Dave
> 
> > 
> > > 
> > > diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> > > index 85fd8d7..fd0905f 100644
> > > --- a/migration/postcopy-ram.c
> > > +++ b/migration/postcopy-ram.c
> > > @@ -64,6 +64,7 @@ static bool ufd_version_check(int ufd)
> > >  {
> > >      struct uffdio_api api_struct;
> > >      uint64_t ioctl_mask;
> > > +    uint64_t features = 0;
> > > 
> > >      api_struct.api = UFFD_API;
> > >      api_struct.features = 0;
> > > @@ -92,6 +93,27 @@ static bool ufd_version_check(int ufd)
> > >              return false;
> > >          }
> > >      }
> > > +
> > > +#ifdef UFFD_FEATURE_THREAD_ID
> > > +    if (api_struct.features & UFFD_FEATURE_THREAD_ID) {
> > > +        features |= UFFD_FEATURE_THREAD_ID;
> > > +    }
> > > +#endif
> > > +
> > > +    if (features) {
> > > +        /*
> > > +         * If there are new features to be enabled from userspace,
> > > +         * trigger another UFFDIO_API ioctl.
> > > +         */
> > > +        api_struct.api = UFFD_API;
> > > +        api_struct.features = features;
> > > +        if (ioctl(ufd, UFFDIO_API, &api_struct)) {
> > > +            error_report("UFFDIO_API failed to setup features: 0x%"PRIx64,
> > > +                         features);
> > > +            return false;
> > > +        }
> > > +    }
> > > +
> > >      return true;
> > >  }
> > > 
> > > > /* only enable the requested features for this uffd context */
> > > >  ctx->features = uffd_ctx_features(features);
> > > > 
> > > > so, at the time when process thread id is going to be sent
> > > > kernel checks if it was requested
> > > > +       if (features & UFFD_FEATURE_THREAD_ID)
> > > > +               msg.arg.pagefault.ptid = task_pid_vnr(current);
> > > 
> > > (I am slightly curious about why we need this if block, after all
> > >  userspace should know whether the ptid field would be valid from the
> > >  first UFFDIO_API ioctl, right?)
> > If I correctly understand you question ) that condition was suggested,
> > due to page faulting is performance critical part (in general, not only postcopy
> > case ), that's why it should be enabled from userspace, 
> > only for statistics/debug purpose.
> > Also looks like David want to see that feature on QEMU as not always
> > feature too.
> > 
> > > 
> > > Thanks,
> > > 
> > > > 
> > > > from patch message:
> > > > 
> > > >  Process's thread id is being provided when user requeste it
> > > > by setting UFFD_FEATURE_THREAD_ID bit into uffdio_api.features.
> > > > 
> > > > UFFD_FEATURE_MISSING_HUGETLBFS - look like default, unconditional
> > > > behavior (I didn't find any usage of that define in kernel).
> > > 
> > > -- 
> > > Peter Xu
> > > 
> > 
> > -- 
> > 
> > BR
> > Alexey
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> 

-- 

BR
Alexey