On 18.02.2015 20:38, Guenter Roeck wrote: > On Tue, Feb 17, 2015 at 03:15:11PM -0800, Andrew Morton wrote: >> On Tue, 17 Feb 2015 20:01:38 +0100 Heinrich Schuchardt <[email protected]> >> 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); > > Apparently it is possible that > mempages < 8 * THREAD_SIZE > which would result in max_threads == 0. > > How about the following ? > > max_threads = mempages / DIV_ROUND_UP(8 * THREAD_SIZE, PAGE_SIZE);
For PAGE_SIZE 2MB and THREAD_SIZE 4kB this is wrong by factor 64. Best regards Heinrich -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
