Re: [PATCH] bcache: fix error count in memory shrink

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Hi Mike---

>Hi Tang Junhui---
>
>I'm not really sure about this one.  It changes the semantics of the
>amount of work done-- nr_to_scan now means number of things to free
>instead of the number to check.
>
The code seems to be designed as that, sc->nr_to_scan marks how much btree
nodes to scan in c->btree_cache_used, and it doesn't include the btree
nodes in c->btree_cache_freeable.

>If the system is under severe memory pressure, and most of the cache is
>essential/actively used, this could greatly increase the amount of work
>done.
>
Yes, I didn't relize this.
>As to the i < btree_cache_used, it's arguably a bug, but it only reduces
>the amount of work done if the cache is very, very small.
>
I think it is a bug, I'll send a new patch to add variable to record 
c->btree_cache_used befor for(;;) loop.

>Mike
>
>On 01/30/2018 06:46 PM, tang.junhui@xxxxxxxxxx wrote:
>> From: Tang Junhui <tang.junhui@xxxxxxxxxx>
>> 
>> In bch_mca_scan(), nr btree nodes are expected to shrink, so the for(;;)
>> loop need to satisfy the condition freed < nr.
>> And since c->btree_cache_used always decrease after mca_data_free() calling
>> in for(;;) loop,  so we need a auto variable to record c->btree_cache_used
>> before the for(;;) loop.
>> 
>> Signed-off-by: Tang Junhui <tang.junhui@xxxxxxxxxx>
>> ---
>>  drivers/md/bcache/btree.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>> 
>> diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
>> index 81e8dc3..7e9ef3d 100644
>> --- a/drivers/md/bcache/btree.c
>> +++ b/drivers/md/bcache/btree.c
>> @@ -664,6 +664,7 @@ static unsigned long bch_mca_scan(struct shrinker *shrink,
>>      struct btree *b, *t;
>>      unsigned long i, nr = sc->nr_to_scan;
>>      unsigned long freed = 0;
>> +    unsigned int btree_cache_used;
>>  
>>      if (c->shrinker_disabled)
>>          return SHRINK_STOP;
>> @@ -700,7 +701,8 @@ static unsigned long bch_mca_scan(struct shrinker *shrink,
>>          }
>>      }
>>  
>> -    for (i = 0; (nr--) && i < c->btree_cache_used; i++) {
>> +    btree_cache_used = c->btree_cache_used;
>> +    for (i = 0; freed < nr && i < btree_cache_used; i++) {
>>          if (list_empty(&c->btree_cache))
>>              goto out;
>>  
>> 
Thanks,
Tang Junhui



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