Re: [PATCH 04/12] mm: add support for async page locking

Jens Axboe
2020-06-01T17:15:27Z

On 6/1/20 8:26 AM, Matthew Wilcox wrote:
> On Tue, May 26, 2020 at 01:51:15PM -0600, Jens Axboe wrote:
>> +static int __wait_on_page_locked_async(struct page *page,
>> + struct wait_page_queue *wait, bool set)
>> +{
>> + struct wait_queue_head *q = page_waitqueue(page);
>> + int ret = 0;
>> +
>> + wait->page = page;
>> + wait->bit_nr = PG_locked;
>> +
>> + spin_lock_irq(&q->lock);
>> + if (set)
>> + ret = !trylock_page(page);
>> + else
>> + ret = PageLocked(page);
>> + if (ret) {
>> + __add_wait_queue_entry_tail(q, &wait->wait);
>> + SetPageWaiters(page);
>> + if (set)
>> + ret = !trylock_page(page);
>> + else
>> + ret = PageLocked(page);
>
> Between the callers and this function, we actually look at PG_lock three
> times; once in the caller, then after taking the spinlock, then after
> adding ourselves to the waitqueue. I understand the first and third, but
> is it really worth doing the second test? It feels unlikely to succeed
> and only saves us setting PageWaiters.

That's probably true, and we can skip the 2nd one. I'll make the change.

--
Jens Axboe