mm/thp: narrow lru locking

lru_lock and page cache xa_lock have no obvious reason to be taken one
way round or the other: until now, lru_lock has been taken before page
cache xa_lock, when splitting a THP; but nothing else takes them
together.  Reverse that ordering: let's narrow the lru locking - but
leave local_irq_disable to block interrupts throughout, like before.

Hugh Dickins point: split_huge_page_to_list() was already silly, to be
using the _irqsave variant: it's just been taking sleeping locks, so
would already be broken if entered with interrupts enabled.  So we can
save passing flags argument down to __split_huge_page().

Why change the lock ordering here? That was hard to decide.  One reason:
when this series reaches per-memcg lru locking, it relies on the THP's
memcg to be stable when taking the lru_lock: that is now done after the
THP's refcount has been frozen, which ensures page memcg cannot change.

Another reason: previously, lock_page_memcg()'s move_lock was presumed
to nest inside lru_lock; but now lru_lock must nest inside (page cache
lock inside) move_lock, so it becomes possible to use lock_page_memcg()
to stabilize page memcg before taking its lru_lock.  That is not the
mechanism used in this series, but it is an option we want to keep open.

[[email protected]: rewrite commit log]

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Alex Shi <[email protected]>
Reviewed-by: Kirill A. Shutemov <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Alexander Duyck <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: "Chen, Rong A" <[email protected]>
Cc: Daniel Jordan <[email protected]>
Cc: "Huang, Ying" <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Konstantin Khlebnikov <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mika Penttilä <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Wei Yang <[email protected]>
Cc: Yang Shi <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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