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. 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.
>
>> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
>
