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Subject: Re: [PATCH 04/12] mm: add support for async page locking
To:     Matthew Wilcox <willy@infradead.org>
Cc:     io-uring@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org
References: <20200526195123.29053-1-axboe@kernel.dk>
 <20200526195123.29053-5-axboe@kernel.dk>
 <20200601142649.GJ19604@bombadil.infradead.org>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <2f4dbb05-4874-6386-f9ee-06fdbaf482bf@kernel.dk>
Date:   Mon, 1 Jun 2020 11:15:22 -0600
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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