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
