linux-kernel - Re: [PATCH 1/2] mm: Fix overcommit overflow

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Message-Id: <20080430140936.9a27facf.akpm@linux-foundation.org>
Date:	Wed, 30 Apr 2008 14:09:36 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: Fix overcommit overflow

On Wed, 30 Apr 2008 21:42:41 +0100
Alan Cox <alan@...rguk.ukuu.org.uk> wrote:

> Sami Farin reported an overflow in the overcommit handling on 64bit boxes.
> 
> We use atomic_t for page counting which is fine on 32bit but on 64bit
> overflows. We can use atomic64_t but there is a problem - most heavy
> users of zero overcommit are embedded people whose 64bit atomics are
> really slow and expensive operations. Thus we use a few defines to flip
> 32 or 64bit according to the size of a long
> 
> (Split into two diffs to keep ownership correct)
> 
> Signed-off-by: Alan Cox <alan@...hat.com>
> 
> diff -u --new-file --recursive --exclude-from /usr/src/exclude linux.vanilla-2.6.25-mm1/include/linux/mman.h linux-2.6.25-mm1/include/linux/mman.h
> --- linux.vanilla-2.6.25-mm1/include/linux/mman.h	2008-04-28 11:35:21.000000000 +0100
> +++ linux-2.6.25-mm1/include/linux/mman.h	2008-04-30 11:21:10.000000000 +0100
> @@ -15,16 +15,31 @@
>  
>  #include <asm/atomic.h>
>  
> +/* 32bit platforms have a virtual address space in pages we
> +   can fit into an atomic_t. We want to avoid atomic64_t on
> +   such boxes as it is often expensive and most strict overcommit
> +   users turn out to be embedded low power processors */
> +
> +#if (BITS_PER_LONG == 32)
> +#define vm_atomic_t atomic_t
> +#define vm_atomic_read atomic_read
> +#define vm_atomic_add atomic_add
> +#else
> +#define vm_atomic_t atomic64_t
> +#define vm_atomic_read atomic64_read
> +#define vm_atomic_add atomic64_add
> +#endif

ew.

>  extern int sysctl_overcommit_memory;
>  extern int sysctl_overcommit_ratio;
> -extern atomic_t vm_committed_space;
> +extern vm_atomic_t vm_committed_space;
>  
>  #ifdef CONFIG_SMP
>  extern void vm_acct_memory(long pages);
>  #else
>  static inline void vm_acct_memory(long pages)
>  {
> -	atomic_add(pages, &vm_committed_space);
> +	vm_atomic_add(pages, &vm_committed_space);
>  }
>  #endif
>  
> diff -u --new-file --recursive --exclude-from /usr/src/exclude linux.vanilla-2.6.25-mm1/mm/mmap.c linux-2.6.25-mm1/mm/mmap.c
> --- linux.vanilla-2.6.25-mm1/mm/mmap.c	2008-04-28 11:36:52.000000000 +0100
> +++ linux-2.6.25-mm1/mm/mmap.c	2008-04-30 11:17:03.000000000 +0100
> @@ -80,7 +80,7 @@
>  int sysctl_overcommit_memory = OVERCOMMIT_GUESS;  /* heuristic overcommit */
>  int sysctl_overcommit_ratio = 50;	/* default is 50% */
>  int sysctl_max_map_count __read_mostly = DEFAULT_MAX_MAP_COUNT;
> -atomic_t vm_committed_space = ATOMIC_INIT(0);
> +vm_atomic_t vm_committed_space = ATOMIC_INIT(0);

That'll need to be ATOMIC64_INIT on 64-bit.

>  
>  /*
>   * Check that a process has enough memory to allocate a new virtual
> @@ -177,7 +177,7 @@
>  	 * cast `allowed' as a signed long because vm_committed_space
>  	 * sometimes has a negative value
>  	 */
> -	if (atomic_read(&vm_committed_space) < (long)allowed)
> +	if (vm_atomic_read(&vm_committed_space) < (long)allowed)
>  		return 0;
>  error:
>  	vm_unacct_memory(pages);
> diff -u --new-file --recursive --exclude-from /usr/src/exclude linux.vanilla-2.6.25-mm1/mm/swap.c linux-2.6.25-mm1/mm/swap.c
> --- linux.vanilla-2.6.25-mm1/mm/swap.c	2008-04-28 11:36:52.000000000 +0100
> +++ linux-2.6.25-mm1/mm/swap.c	2008-04-30 11:18:05.000000000 +0100
> @@ -503,7 +503,7 @@
>  	local = &__get_cpu_var(committed_space);
>  	*local += pages;
>  	if (*local > ACCT_THRESHOLD || *local < -ACCT_THRESHOLD) {
> -		atomic_add(*local, &vm_committed_space);
> +		vm_atomic_add(*local, &vm_committed_space);
>  		*local = 0;
>  	}
>  	preempt_enable();

But afacit the existing atomic_long_t does exactly what you want?

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