Re: [PATCH v1 4/4] Revert "x86/xen: allow nesting of same lazy mode"

[David Hildenbrand <david@redhat.com>]

On 03.03.25 13:33, Ryan Roberts wrote:
> On 03/03/2025 11:52, David Hildenbrand wrote:
>> On 02.03.25 15:55, Ryan Roberts wrote:
>>> Commit 49147beb0ccb ("x86/xen: allow nesting of same lazy mode") was
>>> added as a solution for a core-mm code change where
>>> arch_[enter|leave]_lazy_mmu_mode() started to be called in a nested
>>> manner; see commit bcc6cc832573 ("mm: add default definition of
>>> set_ptes()").
>>>
>>> However, now that we have fixed the API to avoid nesting, we no longer
>>> need this capability in the x86 implementation.
>>>
>>> Additionally, from code review, I don't believe the fix was ever robust
>>> in the case of preemption occurring while in the nested lazy mode. The
>>> implementation usually deals with preemption by calling
>>> arch_leave_lazy_mmu_mode() from xen_start_context_switch() for the
>>> outgoing task if we are in the lazy mmu mode. Then in
>>> xen_end_context_switch(), it restarts the lazy mode by calling
>>> arch_enter_lazy_mmu_mode() for an incoming task that was in the lazy
>>> mode when it was switched out. But arch_leave_lazy_mmu_mode() will only
>>> unwind a single level of nesting. If we are in the double nest, then
>>> it's not fully unwound and per-cpu variables are left in a bad state.
>>>
>>> So the correct solution is to remove the possibility of nesting from the
>>> higher level (which has now been done) and remove this x86-specific
>>> solution.
>>>
>>> Fixes: 49147beb0ccb ("x86/xen: allow nesting of same lazy mode")
>>
>> Does this patch here deserve this tag? IIUC, it's rather a cleanup now that it
>> was properly fixed elsewhere.
> 
> Now that nesting is not possible, yes it is just a cleanup. But when nesting was
> possible, as far as I can tell it was buggy, as per my description.

Right, I understood that part.

> So it's a
> bug bug that won't ever trigger once the other fixes are applied. Happy to
> remove the Fixes and then not include it for stable for v2. That's probably
> simplest.

I was just curious, because it sounded like the actual fix was the other 
patch. Whatever you think is best :)

-- 
Cheers,

David / dhildenb