Re: [PATCH] mm: try to free swap only for reading swap fault

[zhouxianrong <zhouxianrong@huawei.com>]

i mean for reading swap fault try_to_free_swap in do_swap_page
could hurt clean swap cache pages and make them dirty. it affects
reclaim procedure in shrink_page_list and let this function write
out much more these dirty anonymous pages. in fact these dirty
anonymous pages might keep clean originally.

On 2017/11/2 21:22, Michal Hocko wrote:
> On Thu 02-11-17 20:35:19, zhouxianrong@huawei.com wrote:
>> From: zhouxianrong <zhouxianrong@huawei.com>
>>
>> the purpose of this patch is that when a reading swap fault
>> happens on a clean swap cache page whose swap count is equal
>> to one, then try_to_free_swap could remove this page from
>> swap cache and mark this page dirty. so if later we reclaimed
>> this page then we could pageout this page due to this dirty.
>> so i want to allow this action only for writing swap fault.
>>
>> i sampled the data of non-dirty anonymous pages which is no
>> need to pageout and total anonymous pages in shrink_page_list.
>>
>> the results are:
>>
>>         non-dirty anonymous pages     total anonymous pages
>> before  26343                         635218
>> after   36907                         634312
>
> This data is absolutely pointless without describing the workload.
> You patch also stil fails to explain which workloads are going to
> benefit/suffer from the change and why it is a good thing to do in
> general.
>
>> Signed-off-by: zhouxianrong <zhouxianrong@huawei.com>
>> ---
>>  mm/memory.c |    2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index a728bed..5a944fe 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -2999,7 +2999,7 @@ int do_swap_page(struct vm_fault *vmf)
>>  	}
>>
>>  	swap_free(entry);
>> -	if (mem_cgroup_swap_full(page) ||
>> +	if (((vmf->flags & FAULT_FLAG_WRITE) && mem_cgroup_swap_full(page)) ||
>>  	    (vma->vm_flags & VM_LOCKED) || PageMlocked(page))
>>  		try_to_free_swap(page);
>>  	unlock_page(page);
>> --
>> 1.7.9.5
>>
>

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