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

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

[zhouxianrong]

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

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

[Michal Hocko]

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
> 

-- 
Michal Hocko
SUSE Labs

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

[zhouxianrong]

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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