Re: [PATCH 1/1 v2] kernel/fork.c: avoid division by zero

Andrew Morton
2015-02-17T23:15:14Z

On Tue, 17 Feb 2015 20:01:38 +0100 Heinrich Schuchardt <xypron.glpk@gmx.de> wrote:

> PAGE_SIZE is not guaranteed to be equal to or less than 8 times the
> THREAD_SIZE.
>
> E.g. architecture hexagon may have page size 1M and thread size 4096.
>
> This would lead to a division by zero.
>
> The futex implementation assumes that tids fit into the FUTEX_TID_MASK.
> This limits the number of allowable threads.
>
> ...
>
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -74,6 +74,7 @@
> #include <linux/uprobes.h>
> #include <linux/aio.h>
> #include <linux/compiler.h>
> +#include <linux/math64.h>
>
> #include <asm/pgtable.h>
> #include <asm/pgalloc.h>
> @@ -255,6 +256,8 @@ void __init __weak arch_task_cache_init(void) { }
>
> void __init fork_init(unsigned long mempages)
> {
> + u64 temp;

That's a really poor name. We should always try to make names
meaningful. Here, something like "threads" would be better.

> #ifndef CONFIG_ARCH_TASK_STRUCT_ALLOCATOR
> #ifndef ARCH_MIN_TASKALIGN
> #define ARCH_MIN_TASKALIGN L1_CACHE_BYTES
> @@ -273,7 +276,16 @@ void __init fork_init(unsigned long mempages)
> * value: the thread structures can take up at most half
> * of memory.
> */
> - max_threads = mempages / (8 * THREAD_SIZE / PAGE_SIZE);
> + temp = div64_u64((u64) mempages * (u64) PAGE_SIZE,
> + (u64) THREAD_SIZE * 8UL);
> +
> + /*
> + * The futex code assumes that tids fit into the FUTEX_TID_MASK.
> + */
> + if (temp < FUTEX_TID_MASK)
> + max_threads = temp;
> + else
> + max_threads = FUTEX_TID_MASK;

Seems rather complicated. How about

max_threads = mempages / (8 * THREAD_SIZE);
max_threads *= PAGE_SIZE;
max_threads = min(max_threads, FUTEX_TID_MASK);

And while we're there, I do think the comments need a refresh. What
does "the thread structures can take up at most half of memory" mean?
And what's the reasoning behind that "8"? I suggest we just delete all
that and make a new attempt at explaining why the code is this way.