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