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

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Date:   Mon, 1 Jun 2020 07:26:49 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     io-uring@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org
Subject: Re: [PATCH 04/12] mm: add support for async page locking

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.