Sorry, busy with other stuff.
Indicating only what really exists sounds cleaner. But I wonder how we would
want to handle in general orders that are effectively non-existant?
I'm not following your distinction between orders that don't "really exist" and
orders that are "effectively non-existant".
I'm questioning whether there should be a distinction at all. We should
just hide what is either non-existant (not implemented) or non-functional.
I guess the real supported orders are:
anon:
min order: 2
max order: PMD_ORDER
anon-shmem:
min order: 1
max order: MAX_PAGECACHE_ORDER
tmpfs-shmem:
min order: PMD_ORDER <= 11 ? PMD_ORDER : NONE
max order: PMD_ORDER <= 11 ? PMD_ORDER : NONE
file:
min order: 1
max order: MAX_PAGECACHE_ORDER
That's my understanding. But not sure about anon-shmem really supporting
order-1, maybe we do.
But today, controls and stats are exposed for:
anon:
min order: 2
max order: PMD_ORDER
anon-shmem:
min order: 2
max order: PMD_ORDER
tmpfs-shmem:
min order: PMD_ORDER
max order: PMD_ORDER
file:
min order: Nothing yet (this patch proposes 1)
max order: Nothing yet (this patch proposes MAX_PAGECACHE_ORDER)
So I think there is definitely a bug for shmem where the minimum order control
should be order-1 but its currently order-2.
Maybe, did not play with that yet. Likely order-1 will work. (although
probably of questionable use :) )
I also wonder about PUD-order for DAX? We don't currently have a stat/control.
If we wanted to add it in future, if we take the "expose all stats/controls for
all orders" approach, we would end up extending all the way to PUD-order and all
the orders between PMD and PUD would be dummy for all memory types. That really
starts to feel odd, so I still favour only populating what's really supported.
I would go further and say that calling the fsdax thing a THP is
borderline wrong and we should not expose any new toggles for it that way.
It really behaves much more like hugetlb folios that can be PTE-mapped
... we cannot split these things, and they are not allocated from the
buddy. So I wouldn't worry about fsdax for now.
fsdax support for compound pages (now large folios) probably never
should have been glued to any THP toggle.
I propose to fix shmem (extend down to 1, stop at MAX_PAGECACHE_ORDER) and
continue with the approach of "indicating only what really exists" for v2.
Shout if you disagree.
Makes sense.
--
Cheers,
David / dhildenb
