Re: [PATCH RFC 15/21] migration: Teach qemu about minor faults and doublemap

[Juan Quintela]

Peter Xu <peterx@redhat.com> wrote:
> When a ramblock is backed by hugetlbfs and the user specified using
> double-map feature, we trap the faults on these regions using minor mode.
> Teach QEMU about that.
>
> Add some sanity check on the fault flags when receiving a uffd message.
> For minor fault trapped ranges, we should always see the MINOR flag set,
> while when using generic missing faults we should never see it.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>

Reviewed-by: Juan Quintela <quintela@redhat.com>



> -    if (!(reg_struct.ioctls & ((__u64)1 << _UFFDIO_COPY))) {

Does qemu have a macro to do this bitmap handling?

>  {
>      MigrationIncomingState *mis = opaque;
>      struct uffd_msg msg;
> +    uint64_t address;
>      int ret;
>      size_t index;
>      RAMBlock *rb = NULL;
> @@ -945,6 +980,7 @@ static void *postcopy_ram_fault_thread(void *opaque)
>      }
>  
>      while (true) {
> +        bool use_minor_fault, minor_flag;

I think that something on the lines of:
           bool src_minor_fault, dst_minor_fault;

will make things simpler.  Reviewing, I have to go back to definition
place to know which is which.

>          ram_addr_t rb_offset;
>          int poll_result;
>  
> @@ -1022,22 +1058,37 @@ static void *postcopy_ram_fault_thread(void *opaque)
>                  break;
>              }
>  
> -            rb_offset = ROUND_DOWN(rb_offset, migration_ram_pagesize(rb));
> -            
> trace_postcopy_ram_fault_thread_request(msg.arg.pagefault.address,
> -                                                qemu_ram_get_idstr(rb),
> -                                                rb_offset,
> -                                                msg.arg.pagefault.feat.ptid);
> -            mark_postcopy_blocktime_begin(
> -                    (uintptr_t)(msg.arg.pagefault.address),
> -                                msg.arg.pagefault.feat.ptid, rb);
> +            address = ROUND_DOWN(msg.arg.pagefault.address,
> +                                 migration_ram_pagesize(rb));
> +            use_minor_fault = postcopy_use_minor_fault(rb);
> +            minor_flag = !!(msg.arg.pagefault.flags &
> +                            UFFD_PAGEFAULT_FLAG_MINOR);
>  
> +            /*
> +             * Do sanity check on the message flags to make sure this is
> +             * the one we expect to receive.  When using minor fault on
> +             * this ramblock, it should _always_ be set; when not using
> +             * minor fault, it should _never_ be set.
> +             */
> +            if (use_minor_fault ^ minor_flag) {
> +                error_report("%s: Unexpected page fault flags (0x%"PRIx64") "
> +                             "for address 0x%"PRIx64" (mode=%s)", __func__,
> +                             (uint64_t)msg.arg.pagefault.flags,
> +                             (uint64_t)msg.arg.pagefault.address,
> +                             use_minor_fault ? "MINOR" : "MISSING");
> +            }
> +
> +            trace_postcopy_ram_fault_thread_request(
> +                address, qemu_ram_get_idstr(rb), rb_offset,
> +                msg.arg.pagefault.feat.ptid);
> +            mark_postcopy_blocktime_begin(
> +                    (uintptr_t)(address), msg.arg.pagefault.feat.ptid, rb);
>  retry:
>              /*
>               * Send the request to the source - we want to request one
>               * of our host page sizes (which is >= TPS)
>               */
> -            ret = postcopy_request_page(mis, rb, rb_offset,
> -                                        msg.arg.pagefault.address);
> +            ret = postcopy_request_page(mis, rb, rb_offset, address);

This is the only change that I find 'problematic'.
On old code, rb_offset has been ROUND_DOWN, on new code it is not.
On old code we pass msg.arg.pagefault.address, now we use
ROUND_DOW(msg.arg.pagefault.address, mighration_ram_pagesize(rb)).

>              if (ret) {
>                  /* May be network failure, try to wait for recovery */
>                  postcopy_pause_fault_thread(mis);
> @@ -1694,3 +1745,13 @@ void *postcopy_preempt_thread(void *opaque)
>  
>      return NULL;
>  }
> +
> +/*
> + * Whether we should use MINOR fault to trap page faults?  It will be used
> + * when doublemap is enabled on hugetlbfs.  The default value will be
> + * false, which means we'll keep using the legacy MISSING faults.
> + */
> +bool postcopy_use_minor_fault(RAMBlock *rb)
> +{
> +    return migrate_hugetlb_doublemap() && qemu_ram_is_hugetlb(rb);
> +}

Are you planing using this function outside postocpy-ram.c?  Otherwise
if you move up its definition you can make it static and drop the header
change.

Later, Juan.