Re: [PATCH RESEND v6 0/5] migration: reduce time of loading non-iterable vmstate

[Chuang Xu]

Hi, Peter,

On 2023/3/8 上午1:04, Peter Xu wrote:
> On Tue, Mar 07, 2023 at 09:24:31PM +0800, Chuang Xu wrote:
>>> Why do we need address_space_get_flatview_rcu()? I'm not sure whether you
>> address_space_cahce_init() uses address_space_get_flatview() to acquire
>> a ref-ed flatview. If we want to use address_space_to_flatview_rcu() and
>> make the flatview ref-ed, maybe we need to add address_space_get_flatview_rcu()?
>> Or if we use address_space_to_flatview_rcu() directly, then we'll get a
>> unref-ed flatview, I'm not sure wheather it's safe in other cases.. At least
>> I don't want the assertion to be triggered one day.
> No we can't touch that, afaict. It was a real fix (447b0d0b9e) to a real
> bug.
>
> What I meant is we make address_space_get_flatview() always use
> address_space_to_flatview_rcu().

This time I clearly understand what you mean.😁

>
> I think it's safe, if you see the current callers of it (after my patch 1
> fixed version applied):
>
> memory_region_sync_dirty_bitmap[2249] view = address_space_get_flatview(as);
> memory_region_update_coalesced_range[2457] view = address_space_get_flatview(as);
> memory_region_clear_dirty_bitmap[2285] view = address_space_get_flatview(as);
> mtree_info_flatview[3418] view = address_space_get_flatview(as);
> address_space_cache_init[3350] cache->fv = address_space_get_flatview(as);
>
> Case 5 is what we're targeting for which I think it's fine. Logically any
> commit() hook should just use address_space_get_flatview_raw() to reference
> the flatview, but at least address_space_cache_init() is also called in
> non-BQL sections, so _rcu is the only thing we can use here, iiuc..
>
> Case 1-3 is never called during vm load, so I think it's safe.
>
> Case 4 can be accessing stalled flatview but I assume fine since it's just
> for debugging. E.g. if someone tries "info mtree -f" during vm loading
> after your patch applied, then one can see stalled info. I don't think it
> can even happen because so far "info mtree" should also take BQL.. so it'll
> be blocked until vm load completes.
>
> The whole thing is still tricky. I didn't yet make my mind through on how

IIUC, Do you mean that different ways to get flatview are tricky?

> to make it very clean, I think it's slightly beyond what this series does
> and I need some more thinkings (or suggestions from others).
>
As you said, it's slightly beyond what this series does. Maybe it would be
better if we discuss it in a new series and keep this series at v6?
what's your take?

Thanks!