Hugh Dickins <[email protected]> wrote:
>
> In this implementation, the spinlock is tucked inside the struct page of
> the page table page: with a BUILD_BUG_ON in case it overflows - which it
> would in the case of 32-bit PA-RISC with spinlock debugging enabled.
eh? It's going to overflow an unsigned long on x86 too:
typedef struct {
raw_spinlock_t raw_lock;
#if defined(CONFIG_PREEMPT) && defined(CONFIG_SMP)
unsigned int break_lock;
#endif
#ifdef CONFIG_DEBUG_SPINLOCK
unsigned int magic, owner_cpu;
void *owner;
#endif
} spinlock_t;
I think we need a union here.
> +#define __pte_lockptr(page) ((spinlock_t *)&((page)->private))
> +#define pte_lock_init(_page) do { \
> + BUILD_BUG_ON((size_t)(__pte_lockptr((struct page *)0) + 1) > \
> + sizeof(struct page)); \
The above assumes that page.private is the final field in struct page.
That's fragile.
> Splitting the lock is not quite for free: another cacheline access.
> Ideally, I suppose we would use split ptlock only for multi-threaded
> processes on multi-cpu machines; but deciding that dynamically would
> have its own costs. So for now enable it by config, at some number
> of cpus - since the Kconfig language doesn't support inequalities, let
> preprocessor compare that with NR_CPUS. But I don't think it's worth
> being user-configurable: for good testing of both split and unsplit
> configs, split now at 4 cpus, and perhaps change that to 8 later.
I'll make it >= 2 for -mm.
> +#define __pte_lockptr(page) ((spinlock_t *)&((page)->private))
> +#define pte_lock_init(_page) do { \
> + BUILD_BUG_ON((size_t)(__pte_lockptr((struct page *)0) + 1) > \
> + sizeof(struct page)); \
-
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