Re: [PATCH 2/5] mm, highmem: remove useless pool_lock

[JoonSoo Kim]

Hello, Andrew.

2012/10/31 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>:
> On Mon, 29 Oct 2012 04:12:53 +0900
> Joonsoo Kim <js1304@xxxxxxxxx> wrote:
>
>> 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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