Re: [PATCH v3 2/3] spapr: nvdimm: Implement H_SCM_FLUSH hcall

[David Gibson]

On Mon, Mar 29, 2021 at 02:53:47PM +0530, Shivaprasad G Bhat wrote:
> 
> On 3/24/21 8:37 AM, David Gibson wrote:
> > On Tue, Mar 23, 2021 at 09:47:38AM -0400, Shivaprasad G Bhat wrote:
> > > machine vmstate.
> > > 
> > > Signed-off-by: Shivaprasad G Bhat<sbhat@linux.ibm.com>
> > An overal question: surely the same issue must arise on x86 with
> > file-backed NVDIMMs.  How do they handle this case?
> 
> Discussed in other threads..
> 
> ....
> 
> > >   };
> > > @@ -2997,6 +3000,9 @@ static void spapr_machine_init(MachineState 
> > > *machine)
> > >       }
> > >       qemu_cond_init(&spapr->fwnmi_machine_check_interlock_cond);
> > > +    qemu_mutex_init(&spapr->spapr_nvdimm_flush_states_lock);
> > Do you actually need an extra mutex, or can you rely on the BQL?
> 
> I verified BQL is held at all places where it matters in the context of this
> patch.
> 
> Safe to get rid of this extra mutex.
> 
> ...
> 
> > 
> > > +{
> > > +     SpaprMachineState *spapr = SPAPR_MACHINE(qdev_get_machine());
> > > +
> > > +     return (!QLIST_EMPTY(&spapr->pending_flush_states) ||
> > > +             !QLIST_EMPTY(&spapr->completed_flush_states));
> > > +}
> > > +
> > > +static int spapr_nvdimm_pre_save(void *opaque)
> > > +{
> > > +    SpaprMachineState *spapr = SPAPR_MACHINE(qdev_get_machine());
> > > +
> > > +    while (!QLIST_EMPTY(&spapr->pending_flush_states)) {
> > > +        aio_poll(qemu_get_aio_context(), true);
> > Hmm... how long could waiting for all the pending flushes to complete
> > take?  This could add substanially to the guest's migration downtime,
> > couldn't it?
> 
> 
> The time taken depends on the number of dirtied pages and the disk io write
> speed. The number of dirty pages on host is configureable with tunables

Well, sure, I'm just trying to get an order of magnitude here.

> vm.dirty_background_ratio (10% default on Fedora 32, Ubuntu 20.04),
> vm.dirty_ratio(20%) of host memory and|or vm.dirty_expire_centisecs(30
> seconds). So, the host itself would be flushing the mmaped file on its own
> from time to time. For guests using the nvdimms with filesystem, the flushes
> would have come frequently and the number of dirty pages might be
> less. The

I'm not sure that necessarily follows.

> pmem applications can use the nvdimms without a filesystem. And for such
> guests, the chances that a flush request can come from pmem applications at
> the time of migration is less or is random. But, the host would have flushed
> the pagecache on its own when vm.dirty_background_ratio is crossed or
> vm.dirty_expire_centisecs expired. So, the worst case would stands at disk
> io latency for writing the dirtied pages in the last
> vm.dirty_expire_centisecs on host OR latency for writing maximum
> vm.dirty_background_ratio(10%) of host RAM. If you want me to calibrate any
> particular size, scenario and get the numbers please let me know.

Hmm.  I feel like this could still easily be 10s, maybe 100s of
milliseconds, yes?  Given that typical migration downtime caps are
also in the 100s of milliseconds, this still seems troublesome.  Since
this is part of the device migration itself, this flushing will all
happen during the downtime, but won't be factored into the downtime
estimations.

> ...
> > > +
> > > +/*
> > > + * Acquire a unique token and reserve it for the new flush state.
> > > + */
> > > +static SpaprNVDIMMDeviceFlushState 
> > > *spapr_nvdimm_init_new_flush_state(void)
> > > +{
> > > +    Error *err = NULL;
> > > +    uint64_t token;
> > > +    SpaprMachineState *spapr = SPAPR_MACHINE(qdev_get_machine());
> > > +    SpaprNVDIMMDeviceFlushState *tmp, *next, *state;
> > > +
> > > +    state = g_malloc0(sizeof(*state));
> > > +
> > > +    qemu_mutex_lock(&spapr->spapr_nvdimm_flush_states_lock);
> > > +retry:
> > > +    if (qemu_guest_getrandom(&token, sizeof(token), &err) < 0) {
> > Using getrandom seems like overkill, why not just use a counter?
> 
> I didnt want a spurious guest to abuse by consuming the return value
> providing
> 
> a valid "guess-able" counter and the real driver failing
> subsequently.

Why would a guessable value be bad?  The counter would be per-guest,
so AFAICT all a malicious guest could do is mess itself up.

> Also,
> across
> 
> guest migrations carrying the global counter to destination is another thing
> to ponder.

I don't think you need to: if there are pending flushes on migration
you can set the dest counter to the max id of those, if not you can
reset it to 1 without harm.

Actually.. come to think of it, can't you just use the current max id
of pending flushes + 1 as a new id.


> Let me know if you want me to reconsider using counter.
> 
> ...
> 
> > > mm_flush_states_lock);
> > > +
> > > +    return state;
> > > +}
> > > +
> > > +/*
> > > + * spapr_nvdimm_finish_flushes
> > > + *      Waits for all pending flush requests to complete
> > > + *      their execution and free the states
> > > + */
> > > +void spapr_nvdimm_finish_flushes(void)
> > > +{
> > > +    SpaprNVDIMMDeviceFlushState *state, *next;
> > > +    SpaprMachineState *spapr = SPAPR_MACHINE(qdev_get_machine());
> > The caller has natural access to the machine, so pass it in rather
> > than using the global.
> 
> okay
> 
> ...
> 
> > > +
> > > +/*
> > > + * spapr_nvdimm_get_hcall_status
> > > + *      Fetches the status of the hcall worker and returns H_BUSY
> > > + *      if the worker is still running.
> > > + */
> > > +static int spapr_nvdimm_get_flush_status(uint64_t token)
> > > +{
> > > +    int ret = H_LONG_BUSY_ORDER_10_MSEC;
> > > +    SpaprMachineState *spapr = SPAPR_MACHINE(qdev_get_machine());
> > The callers have natural access to spapr, so pass it in rather than
> > using the global.
> 
> Okay
> 
> ...
> 
> > > +
> > > +/*
> > > + * H_SCM_FLUSH
> > > + * Input: drc_index, continue-token
> > > + * Out: continue-token
> > > + * Return Value: H_SUCCESS, H_Parameter, H_P2, H_BUSY
> > > + *
> > > + * Given a DRC Index Flush the data to backend NVDIMM device.
> > > + * The hcall returns H_BUSY when the flush takes longer time and the 
> > > hcall
> > It returns one of the H_LONG_BUSY values, not actual H_BUSY, doesn't
> > it?
> 
> Yes. I thought its okay to call it just H_BUSY in a generic way. Will fix
> it.
> 
> 
> > > + * needs to be issued multiple times in order to be completely serviced.
> > > +        }
> > > +
> > > +        return ret;
> > > +    }
> > > +
> > > +    dimm = PC_DIMM(drc->dev);
> > > +    backend = MEMORY_BACKEND(dimm->hostmem);
> > > +
> > > +    state = spapr_nvdimm_init_new_flush_state();
> > > +    if (!state) {
> > > +        return H_P2;
> > AFAICT the only way init_new_flush_state() fails is a failure in the
> > RNG, which definitely isn't a parameter problem.
> 
> Will change it to H_HARDWARE.
> 
> 
> > > +    }
> > > +
> > > +    state->backend_fd = memory_region_get_fd(&backend->mr);
> > Is this guaranteed to return a usable fd in all configurations?
> 
> Right, for memory-backend-ram this wont work. I think we should
> 
> not set the hcall-flush-required too for memory-backend-ram. Will fix this.

Right, but it's good to be defensive here.  I think a bad guest could
initiate this path even if it's not supposed to yes?

> > > +    thread_pool_submit_aio(pool, flush_worker_cb, state,
> > > +                           spapr_nvdimm_flush_completion_cb, state);
> > > +
> > > +    ret = spapr_nvdimm_get_flush_status(state->continue_token);
> > > +    if (H_IS_LONG_BUSY(ret)) {
> > > +        args[0] = state->continue_token;
> > > +    }
> > > +
> > > +    return ret;
> > I believe you can rearrange this so the get_flush_status / check /
> > return is shared between the args[0] == 0 and args[0] == token paths.
> okay.
> 
> Thanks,
> 
> Shiva
> 

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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