linux-kernel - Re: [PATCH 2/5] mm, highmem: remove useless pool

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Message-Id: <20121030143107.ee1f959b.akpm@linux-foundation.org>
Date:	Tue, 30 Oct 2012 14:31:07 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Joonsoo Kim <js1304@...il.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 2/5] mm, highmem: remove useless pool_lock

On Mon, 29 Oct 2012 04:12:53 +0900
Joonsoo Kim <js1304@...il.com> 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.

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.

ARCH_NEEDS_KMAP_HIGH_GET is a nasty looking thing.  It's ARM:

/*
 * The reason for kmap_high_get() is to ensure that the currently kmap'd
 * page usage count does not decrease to zero while we're using its
 * existing virtual mapping in an atomic context.  With a VIVT cache this
 * is essential to do, but with a VIPT cache this is only an optimization
 * so not to pay the price of establishing a second mapping if an existing
 * one can be used.  However, on platforms without hardware TLB maintenance
 * broadcast, we simply cannot use ARCH_NEEDS_KMAP_HIGH_GET at all since
 * the locking involved must also disable IRQs which is incompatible with
 * the IPI mechanism used by global TLB operations.
 */
#define ARCH_NEEDS_KMAP_HIGH_GET
#if defined(CONFIG_SMP) && defined(CONFIG_CPU_TLB_V6)
#undef ARCH_NEEDS_KMAP_HIGH_GET
#if defined(CONFIG_HIGHMEM) && defined(CONFIG_CPU_CACHE_VIVT)
#error "The sum of features in your kernel config cannot be supported together"
#endif
#endif

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