question on NUMA page migration

From: Rik van Riel
Date: Fri Oct 19 2012 - 11:53:40 EST

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
a put_page:

* 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? :)

All rights reversed
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at