Re: [PATCH] mm: filemap: clear idle flag for writes

[Yang Shi <yang.shi@linux.alibaba.com>]

On 6/24/20 11:53 AM, Andrew Morton wrote:
> On Thu, 25 Jun 2020 01:43:32 +0800 Yang Shi <yang.shi@linux.alibaba.com> wrote:
>
>> Since commit bbddabe2e436aa7869b3ac5248df5c14ddde0cbf ("mm: filemap:
>> only do access activations on reads"), mark_page_accessed() is called
>> for reads only.  But the idle flag is cleared by mark_page_accessed() so
>> the idle flag won't get cleared if the page is write accessed only.
>>
>> Basically idle page tracking is used to estimate workingset size of
>> workload, noticeable size of workingset might be missed if the idle flag
>> is not maintained correctly.
>>
>> It seems good enough to just clear idle flag for write operations.
>>
>> ...
>>
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -41,6 +41,7 @@
>>   #include <linux/delayacct.h>
>>   #include <linux/psi.h>
>>   #include <linux/ramfs.h>
>> +#include <linux/page_idle.h>
>>   #include "internal.h"
>>   
>>   #define CREATE_TRACE_POINTS
>> @@ -1630,6 +1631,11 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t index,
>>   
>>   	if (fgp_flags & FGP_ACCESSED)
>>   		mark_page_accessed(page);
>> +	else if (fgp_flags & FGP_WRITE) {
>> +		/* Clear idle flag for buffer write */
>> +		if (page_is_idle(page))
>> +			clear_page_idle(page);
>> +	}
>>   
>>   no_page:
>>   	if (!page && (fgp_flags & FGP_CREAT)) {
> The kerneldoc comment for pagecache_get_page() could do with some
> updating - it fails to mention FGP_WRITE, FGP_NOFS and FGP_NOWAIT.

Yes, will propose a separate patch later on.

>
> This change seems correct but also will have runtime effects.  What are
> they?

Other than a couple of extra cycles when idle page tracking is enabled, 
I didn't think of other effects. It should be negligible. The idle flag 
doesn't play a role in page reclaim algorithm, so it won't have impact 
on that.