question on NUMA page migration

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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.
         */
        put_page(page);

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? :)

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