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

[Juan Quintela]

Peter Xu <peterx@redhat.com> wrote:
> On Mon, Jan 30, 2023 at 06:45:20AM +0100, Juan Quintela wrote:
>> 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?
>
> Not yet that's suitable.  It's open coded like this in many places of
> postcopy.  One thing close enough is bitmap_test_and_clear() but too heavy.
>
>> 
>> >  {
>> >      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.
>
> These two values represents "what we expect" and "what we got from the
> message", so the only thing is I'm not sure whether src/dst matches the
> best here.
>
> How about "expect_minor_fault" and "has_minor_fault" instead?

Perfect with me.

>> >              /*
>> >               * 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)).
>
> Thanks for spotting such a detail even for a RFC series. :)
>
> It's actually rounded down to target psize, here since we're in postcopy we
> should require target psize equals to host psize (or I bet it won't really
> work at all).  So the relevant rounddown was actually done here:
>
>             rb = qemu_ram_block_from_host(
>                      (void *)(uintptr_t)msg.arg.pagefault.address,
>                      true, &rb_offset);
>
> In which there's:
>
>     *offset = (host - block->host);
>     if (round_offset) {
>         *offset &= TARGET_PAGE_MASK;
>     }
>
> So when I rework that chunk of code I directly dropped the ROUND_DOWN()
> because I find it duplicated.

Ok.

>
>> 
>> >              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.
>
> Yes, it'll be further used in ram.c later in the patch "migration: Rework
> ram discard logic for hugetlb double-map" right below.

Aha.

Thanks.