Re: [RFC V3 PATCH] arm64: mm: swap: save and restore mte tags for large folios

[David Hildenbrand]

> > > Are you referring to Steven's suggestion of reading the tag to see if it's
> > > zeros? I think that demonstrates my point that this has to be done per-page and
> >
> > Yes.
>
> OK I'm obviously being thick, because checking a page's tag to see if its zero
> seems logically equivalent to checking the existing per-page flag, just more
> expensive. Yes we could make that change but I don't see how it helps solve the
> real problem at hand. Unless you are also deliberately trying to remove the
> per-page flag at the same time, as per Matthew's master plan?

I think a per-folio interface is cleaner and more future-proof, and 
removing the per-page flag might be a nice side product of that.

Anyhow, just some thoughts from my side if it could be easily/cleanly done.

At least the "easy" part does not seem to be the case, so I'm fine with 
deferring anything like that for now.

[...]

> > We can identify in 1) the subpages by reading the tag from HW,
>
> I don't think this actually works; I'm pretty sure the optimization to clear the
> tag at the same time as the page clearing only happens for small pages. I don't
> think this will be done when allocating a THP today. Obviously that could change.

Could be, absolutely no expert. I was primarily on the "It would be 
possible to reverse this scheme - we could drop the page
flag and just look at the actual tag storage. If it's all zeros then
obviously there's no point in storing it." comment from Steven.

> > and on 2) by
> > checking the datastructure. For 3), there is nothing to check.
> >
> > On swapout of a large folio:
> >
> > * For 3) we don't do anything
> > * For 2) we don't do anything
> > * For 1) we store all tags that are non-zero (reading all tags) and
> >    transition to 2).
>
> Given a tag architecturally exists for every page even when unused, and we think
> a folio being partially mte-tagged is the corner case, could you simplify this
> further and just write out all the tags for the folio and not care if some are
> not in use?

Likely this could be simplified, yes.

--
Cheers,

David / dhildenb