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SubjectRe: [PATCH 1/2] mm: Fix overcommit overflow
On Wed, 30 Apr 2008 21:42:41 +0100
Alan Cox <alan@lxorguk.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@redhat.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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