Re: [RFC PATCH v2 13/16] mm: add balance_dirty_pages_ratelimited_flags() function

[Stefan Roesch <shr@fb.com>]

On 5/17/22 1:12 PM, Jan Kara wrote:
> On Mon 16-05-22 09:47:15, Stefan Roesch wrote:
>> This adds the function balance_dirty_pages_ratelimited_flags(). It adds
>> the parameter is_async to balance_dirty_pages_ratelimited(). In case
>> this is an async write, it will call _balance_diirty_pages() to
>> determine if write throttling needs to be enabled. If write throttling
>> is enabled, it retuns -EAGAIN, so the write request can be punted to
>> the io-uring worker.
>>
>> The new function is changed to return the sleep time, so callers can
>> observe if the write has been punted.
>>
>> For non-async writes the current behavior is maintained.
>>
>> Signed-off-by: Stefan Roesch <shr@fb.com>
>> ---
>>  include/linux/writeback.h |  1 +
>>  mm/page-writeback.c       | 48 ++++++++++++++++++++++++++-------------
>>  2 files changed, 33 insertions(+), 16 deletions(-)
>>
>> diff --git a/include/linux/writeback.h b/include/linux/writeback.h
>> index fec248ab1fec..d589804bb3be 100644
>> --- a/include/linux/writeback.h
>> +++ b/include/linux/writeback.h
>> @@ -373,6 +373,7 @@ unsigned long wb_calc_thresh(struct bdi_writeback *wb, unsigned long thresh);
>>  
>>  void wb_update_bandwidth(struct bdi_writeback *wb);
>>  void balance_dirty_pages_ratelimited(struct address_space *mapping);
>> +int  balance_dirty_pages_ratelimited_flags(struct address_space *mapping, bool is_async);
>>  bool wb_over_bg_thresh(struct bdi_writeback *wb);
>>  
>>  typedef int (*writepage_t)(struct page *page, struct writeback_control *wbc,
>> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
>> index cbb74c0666c6..78f1326f3f20 100644
>> --- a/mm/page-writeback.c
>> +++ b/mm/page-writeback.c
>> @@ -1877,28 +1877,17 @@ static DEFINE_PER_CPU(int, bdp_ratelimits);
>>   */
>>  DEFINE_PER_CPU(int, dirty_throttle_leaks) = 0;
>>  
>> -/**
>> - * balance_dirty_pages_ratelimited - balance dirty memory state
>> - * @mapping: address_space which was dirtied
>> - *
>> - * Processes which are dirtying memory should call in here once for each page
>> - * which was newly dirtied.  The function will periodically check the system's
>> - * dirty state and will initiate writeback if needed.
>> - *
>> - * Once we're over the dirty memory limit we decrease the ratelimiting
>> - * by a lot, to prevent individual processes from overshooting the limit
>> - * by (ratelimit_pages) each.
>> - */
>> -void balance_dirty_pages_ratelimited(struct address_space *mapping)
>> +int balance_dirty_pages_ratelimited_flags(struct address_space *mapping, bool is_async)
> 
> Perhaps I'd call the other function balance_dirty_pages_ratelimited_async()
> and then keep balance_dirty_pages_ratelimited_flags() as an internal
> function. It is then more obvious at the external call sites what the call
> is about (unlike the true/false argument).
> 

I added a new function balance_dirty_pages_ratelimited_async() and made the function
balance_dirty_pages_ratelimited_flags an internal function.

> 								Honza
> 
>>  {
>>  	struct inode *inode = mapping->host;
>>  	struct backing_dev_info *bdi = inode_to_bdi(inode);
>>  	struct bdi_writeback *wb = NULL;
>>  	int ratelimit;
>> +	int ret = 0;
>>  	int *p;
>>  
>>  	if (!(bdi->capabilities & BDI_CAP_WRITEBACK))
>> -		return;
>> +		return ret;
>>  
>>  	if (inode_cgwb_enabled(inode))
>>  		wb = wb_get_create_current(bdi, GFP_KERNEL);
>> @@ -1937,10 +1926,37 @@ void balance_dirty_pages_ratelimited(struct address_space *mapping)
>>  	}
>>  	preempt_enable();
>>  
>> -	if (unlikely(current->nr_dirtied >= ratelimit))
>> -		balance_dirty_pages(wb, current->nr_dirtied);
>> +	if (unlikely(current->nr_dirtied >= ratelimit)) {
>> +		if (is_async) {
>> +			struct bdp_ctx ctx = { BDP_CTX_INIT(ctx, wb) };
>> +
>> +			ret = _balance_dirty_pages(wb, current->nr_dirtied, &ctx);
>> +			if (ret)
>> +				ret = -EAGAIN;
>> +		} else {
>> +			balance_dirty_pages(wb, current->nr_dirtied);
>> +		}
>> +	}
>>  
>>  	wb_put(wb);
>> +	return ret;
>> +}
>> +
>> +/**
>> + * balance_dirty_pages_ratelimited - balance dirty memory state
>> + * @mapping: address_space which was dirtied
>> + *
>> + * Processes which are dirtying memory should call in here once for each page
>> + * which was newly dirtied.  The function will periodically check the system's
>> + * dirty state and will initiate writeback if needed.
>> + *
>> + * Once we're over the dirty memory limit we decrease the ratelimiting
>> + * by a lot, to prevent individual processes from overshooting the limit
>> + * by (ratelimit_pages) each.
>> + */
>> +void balance_dirty_pages_ratelimited(struct address_space *mapping)
>> +{
>> +	balance_dirty_pages_ratelimited_flags(mapping, false);
>>  }
>>  EXPORT_SYMBOL(balance_dirty_pages_ratelimited);
>>  
>> -- 
>> 2.30.2
>>