Hi Andrea, Peter,
I have a question on page refcounting in your NUMA
page migration code.
In Peter's case, I wonder why you introduce a new
MIGRATE_FAULT migration mode. If the normal page
migration / compaction logic can do without taking
an extra reference count, why does your code need it?
In Andrea's case, we have a comment suggesting an
extra refcount is needed, immediately followed by
* Pin the head subpage at least until the first
* __isolate_lru_page succeeds (__isolate_lru_page pins it
* again when it succeeds). If we unpin before
* __isolate_lru_page successd, the page could be freed and
* reallocated out from under us. Thus our previous checks on
* the page, and the split_huge_page, would be worthless.
* We really only need to do this if "ret > 0" but it doesn't
* hurt to do it unconditionally as nobody can reference
* "page" anymore after this and so we can avoid an "if (ret >
* 0)" branch here.
This also confuses me.
If we do not need the extra refcount (and I do not
understand why NUMA migrate-on-fault needs one more
refcount than normal page migration), we can get
rid of the MIGRATE_FAULT mode.
If we do need the extra refcount, why is normal
page migration safe? :)