The one thing we absolutely cannot have is a default that causes this
'creeping' behaviour. This feels like shipping something that is broken and
alluding to it in the documentation.
I spoke to David off-list and he gave some insight into this and perhaps
some reasonable means of avoiding an additional tunable.
I don't want to rehash what he said as I think it's more productive for him
to reply when he has time but broadly I think how we handle this needs
careful consideration.
To me it's clear that some sense of ratio is just immediately very very
confusing, but then again this interface is already confusing, as with much
of THP.
Anyway I'll let David respond here so we don't loop around before he has a
chance to add his input.
Cheers, Lorenzo
[Resending because Thunderbird decided to use the wrong smtp server]
I've been summoned.
As raised in the past, I would initially only support specific values here like
0 : Never collapse with any pte_none/zeropage
511 (HPAGE_PMD_NR - 1) / default : Always collapse, ignoring pte_none/zeropage
Once could also easily support the value 255 (HPAGE_PMD_NR / 2- 1), but not sure
if we have to add that for now.
Because, as raised in the past, I'm afraid nobody on this earth has a clue how
to set this parameter to values different to 0 (don't waste memory with khugepaged)
and 511 (page fault behavior).
If any other value is set, essentially
pr_warn("Unsupported 'max_ptes_none' value for mTHP collapse");
for now and just disable it.
--
Cheers
David / dhildenb
