Re: [PATCH] mm, page_alloc: reset the zone->watermark_boost early

[Charan Teja Kalla]

Thank you Andrew for the reply.

On 5/12/2020 1:41 AM, Andrew Morton wrote:
> On Mon, 11 May 2020 19:10:08 +0530 Charan Teja Reddy <charante@xxxxxxxxxxxxxx> wrote:
> 
>> Updating the zone watermarks by any means, like extra_free_kbytes,
>> min_free_kbytes, water_mark_scale_factor e.t.c, when watermark_boost is
>> set will result into the higher low and high watermarks than the user
>> asks. This can be avoided by resetting the zone->watermark_boost to zero
>> early.
> 
> Does this solve some problem which has been observed in testing?

Sorry, what are those issues observed in testing? It would be helpful
If you post them here. 

> 
>> ...
>>
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -7746,9 +7746,9 @@ static void __setup_per_zone_wmarks(void)
>>  			    mult_frac(zone_managed_pages(zone),
>>  				      watermark_scale_factor, 10000));
>>  
>> +		zone->watermark_boost = 0;
>>  		zone->_watermark[WMARK_LOW]  = min_wmark_pages(zone) + tmp;
>>  		zone->_watermark[WMARK_HIGH] = min_wmark_pages(zone) + tmp * 2;
>> -		zone->watermark_boost = 0;
>>  
>>  		spin_unlock_irqrestore(&zone->lock, flags);
>>  	}
> 
> This could only be a problem if code is accessing these things without
> holding zone->lock.  Is that ever the case?
> 

This is a problem even when accessing these things with zone->lock
held because we are directly using the macro min_wmark_pages(zone)
which leads to the issue. Pasting macro here for reference.

#define min_wmark_pages(z) (z->_watermark[WMARK_MIN] + z->watermark_boost)

Steps that lead to the issue is like below:
1) On the extfrag event, we try to boost the watermark by storing the
   value in ->watermark_boost.

2) User changes the value of extra|min_free_kbytes or watermark_scale_factor.
  
   In __setup_perzone_wmarks, we directly store the user asked
   watermarks in the zones structure. In this step, the value
   is always offsets by ->watermark_boost as we use the min_wmark_pages() macro.

3) Later, when kswapd woke up, it resets the zone's watermark_boost to zero. 

Step 2 from the above is what resulting into the issue.

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