Hello, Andrew.

2012/10/31 Andrew Morton [off-list ref]:

On Mon, 29 Oct 2012 04:12:53 +0900
Joonsoo Kim [off-list ref] wrote:

quoted

The pool_lock protects the page_address_pool from concurrent access.
But, access to the page_address_pool is already protected by kmap_lock.
So remove it.

Well, there's a set_page_address() call in mm/page_alloc.c which
doesn't have lock_kmap().  it doesn't *need* lock_kmap() because it's
init-time code and we're running single-threaded there.  I hope!

But this exception should be double-checked and mentioned in the
changelog, please.  And it's a reason why we can't add
assert_spin_locked(&kmap_lock) to set_page_address(), which is
unfortunate.

set_page_address() in mm/page_alloc.c is invoked only when
WANT_PAGE_VIRTUAL is defined.
And in this case, set_page_address()'s definition is not in highmem.c,
but in include/linux/mm.h.
So, we don't need to worry about set_page_address() call in mm/page_alloc.c

The irq-disabling in this code is odd.  If ARCH_NEEDS_KMAP_HIGH_GET=n,
we didn't need irq-safe locking in set_page_address().  I guess we'll
need to retain it in page_address() - I expect some callers have IRQs
disabled.

As Minchan described, if we don't disable irq when we take a lock for pas->lock,
it would be deadlock with page_address().

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