Re: [PATCH 7.2 v10 1/2] mm/mmu_gather: prepare to skip redundant sync IPIs

[Dave Hansen <dave.hansen@intel.com>]

On 4/24/26 08:04, Peter Zijlstra wrote:
> So I don't like this at all.... The comment says there is a preceding
> TLB flush, but there is nothing that guarantees there is. One would have
> to go audit all users and ensure this is always true.
> 
> This thing is incredibly fragile.

Yeah, this seems like an attempt to apply a code solution to a data
structure problem.

I think I talked about this in earlier iterations. But, ideally, what
happens here is that the things doing the table freeing or collapsing or
whatever would note in a data structure what they did.

Then the actual flushing code can look at the data structure and figure
out what kind of flush it needs. Things like "do I need to flush on lazy
CPUs?" Or, "have I done an IPI since the last page table free?"

But, if I remember from earlier in this thread, some of the callers of
this stuff didn't have a nice data structure (like an mmu_gather) passed
in to the places where it would be needed to exfiltrate the information.

I think Lance gave up on that because it looked too invasive to him.

But, I think this boils down to the code being too fragile as-is to
support what Lance is trying to do. It actually needs some refactoring
love before it can support the desired optimization. I'm not sure
there's an easy way out here.