Re: [PATCH v2 1/4] mm/filemap: Add folio_lock_timeout()

[Mel Gorman <mgorman@techsingularity.net>]

On Fri, Apr 21, 2023 at 03:12:45PM -0700, Douglas Anderson wrote:
> Add a variant of folio_lock() that can timeout. This is useful to
> avoid unbounded waits for the page lock in kcompactd.
> 
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
> 
> Changes in v2:
> - "Add folio_lock_timeout()" new for v2.
> 
>  include/linux/pagemap.h | 16 ++++++++++++++
>  mm/filemap.c            | 47 +++++++++++++++++++++++++++++------------
>  2 files changed, 50 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 0acb8e1fb7af..0f3ef9f79300 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -892,6 +892,7 @@ static inline bool wake_page_match(struct wait_page_queue *wait_page,
>  }
>  
>  void __folio_lock(struct folio *folio);
> +int __folio_lock_timeout(struct folio *folio, long timeout);
>  int __folio_lock_killable(struct folio *folio);
>  bool __folio_lock_or_retry(struct folio *folio, struct mm_struct *mm,
>  				unsigned int flags);
> @@ -952,6 +953,21 @@ static inline void folio_lock(struct folio *folio)
>  		__folio_lock(folio);
>  }
>  
> +/**
> + * folio_lock_timeout() - Lock this folio, with a timeout.
> + * @folio: The folio to lock.
> + * @timeout: The timeout in jiffies; %MAX_SCHEDULE_TIMEOUT means wait forever.
> + *
> + * Return: 0 upon success; -ETIMEDOUT upon failure.
> + */

May return -EINTR?

> +static inline int folio_lock_timeout(struct folio *folio, long timeout)
> +{
> +	might_sleep();
> +	if (!folio_trylock(folio))
> +		return __folio_lock_timeout(folio, timeout);
> +	return 0;
> +}
> +
>  /**
>   * lock_page() - Lock the folio containing this page.
>   * @page: The page to lock.
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 2723104cc06a..c6056ec41284 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1220,7 +1220,7 @@ static inline bool folio_trylock_flag(struct folio *folio, int bit_nr,
>  int sysctl_page_lock_unfairness = 5;
>  
>  static inline int folio_wait_bit_common(struct folio *folio, int bit_nr,
> -		int state, enum behavior behavior)
> +		int state, enum behavior behavior, long timeout)
>  {
>  	wait_queue_head_t *q = folio_waitqueue(folio);
>  	int unfairness = sysctl_page_lock_unfairness;
> @@ -1229,6 +1229,7 @@ static inline int folio_wait_bit_common(struct folio *folio, int bit_nr,
>  	bool thrashing = false;
>  	unsigned long pflags;
>  	bool in_thrashing;
> +	int err;
>  
>  	if (bit_nr == PG_locked &&
>  	    !folio_test_uptodate(folio) && folio_test_workingset(folio)) {
> @@ -1295,10 +1296,13 @@ static inline int folio_wait_bit_common(struct folio *folio, int bit_nr,
>  		/* Loop until we've been woken or interrupted */
>  		flags = smp_load_acquire(&wait->flags);
>  		if (!(flags & WQ_FLAG_WOKEN)) {
> +			if (!timeout)
> +				break;
> +

An io_schedule_timeout of 0 is valid so why the special handling? It's
negative timeouts that cause schedule_timeout() to complain.

>  			if (signal_pending_state(state, current))
>  				break;
>  
> -			io_schedule();
> +			timeout = io_schedule_timeout(timeout);
>  			continue;
>  		}
>  
> @@ -1324,10 +1328,10 @@ static inline int folio_wait_bit_common(struct folio *folio, int bit_nr,
>  	}
>  
>  	/*
> -	 * If a signal happened, this 'finish_wait()' may remove the last
> -	 * waiter from the wait-queues, but the folio waiters bit will remain
> -	 * set. That's ok. The next wakeup will take care of it, and trying
> -	 * to do it here would be difficult and prone to races.
> +	 * If a signal/timeout happened, this 'finish_wait()' may remove the
> +	 * last waiter from the wait-queues, but the folio waiters bit will
> +	 * remain set. That's ok. The next wakeup will take care of it, and
> +	 * trying to do it here would be difficult and prone to races.
>  	 */
>  	finish_wait(q, wait);
>  
> @@ -1336,6 +1340,13 @@ static inline int folio_wait_bit_common(struct folio *folio, int bit_nr,
>  		psi_memstall_leave(&pflags);
>  	}
>  
> +	/*
> +	 * If we don't meet the success criteria below then we've got an error
> +	 * of some sort. Differentiate between the two error cases. If there's
> +	 * no time left it must have been a timeout.
> +	 */
> +	err = !timeout ? -ETIMEDOUT : -EINTR;
> +
>  	/*
>  	 * NOTE! The wait->flags weren't stable until we've done the
>  	 * 'finish_wait()', and we could have exited the loop above due
> @@ -1350,9 +1361,9 @@ static inline int folio_wait_bit_common(struct folio *folio, int bit_nr,
>  	 * waiter, but an exclusive one requires WQ_FLAG_DONE.
>  	 */
>  	if (behavior == EXCLUSIVE)
> -		return wait->flags & WQ_FLAG_DONE ? 0 : -EINTR;
> +		return wait->flags & WQ_FLAG_DONE ? 0 : err;
>  
> -	return wait->flags & WQ_FLAG_WOKEN ? 0 : -EINTR;
> +	return wait->flags & WQ_FLAG_WOKEN ? 0 : err;
>  }
>  
>  #ifdef CONFIG_MIGRATION
> @@ -1442,13 +1453,15 @@ void migration_entry_wait_on_locked(swp_entry_t entry, pte_t *ptep,
>  
>  void folio_wait_bit(struct folio *folio, int bit_nr)
>  {
> -	folio_wait_bit_common(folio, bit_nr, TASK_UNINTERRUPTIBLE, SHARED);
> +	folio_wait_bit_common(folio, bit_nr, TASK_UNINTERRUPTIBLE, SHARED,
> +			      MAX_SCHEDULE_TIMEOUT);
>  }
>  EXPORT_SYMBOL(folio_wait_bit);
>  
>  int folio_wait_bit_killable(struct folio *folio, int bit_nr)
>  {
> -	return folio_wait_bit_common(folio, bit_nr, TASK_KILLABLE, SHARED);
> +	return folio_wait_bit_common(folio, bit_nr, TASK_KILLABLE, SHARED,
> +				     MAX_SCHEDULE_TIMEOUT);
>  }
>  EXPORT_SYMBOL(folio_wait_bit_killable);
>  
> @@ -1467,7 +1480,8 @@ EXPORT_SYMBOL(folio_wait_bit_killable);
>   */
>  static int folio_put_wait_locked(struct folio *folio, int state)
>  {
> -	return folio_wait_bit_common(folio, PG_locked, state, DROP);
> +	return folio_wait_bit_common(folio, PG_locked, state, DROP,
> +				     MAX_SCHEDULE_TIMEOUT);
>  }
>  
>  /**
> @@ -1662,17 +1676,24 @@ EXPORT_SYMBOL_GPL(page_endio);
>  void __folio_lock(struct folio *folio)
>  {
>  	folio_wait_bit_common(folio, PG_locked, TASK_UNINTERRUPTIBLE,
> -				EXCLUSIVE);
> +				EXCLUSIVE, MAX_SCHEDULE_TIMEOUT);
>  }
>  EXPORT_SYMBOL(__folio_lock);
>  
>  int __folio_lock_killable(struct folio *folio)
>  {
>  	return folio_wait_bit_common(folio, PG_locked, TASK_KILLABLE,
> -					EXCLUSIVE);
> +					EXCLUSIVE, MAX_SCHEDULE_TIMEOUT);
>  }
>  EXPORT_SYMBOL_GPL(__folio_lock_killable);
>  
> +int __folio_lock_timeout(struct folio *folio, long timeout)
> +{
> +	return folio_wait_bit_common(folio, PG_locked, TASK_KILLABLE,
> +					EXCLUSIVE, timeout);
> +}
> +EXPORT_SYMBOL_GPL(__folio_lock_timeout);
> +
>  static int __folio_lock_async(struct folio *folio, struct wait_page_queue *wait)
>  {
>  	struct wait_queue_head *q = folio_waitqueue(folio);
> -- 
> 2.40.0.634.g4ca3ef3211-goog
> 

-- 
Mel Gorman
SUSE Labs