Re: [PATCH 4/5] mm: simplify reclaim path for MADV_FREE

[yalin wang]

> On Oct 27, 2015, at 16:10, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> 
> On Tue, Oct 27, 2015 at 03:39:16PM +0800, yalin wang wrote:
>> 
>>> On Oct 27, 2015, at 15:09, Minchan Kim <minchan@xxxxxxxxxx> wrote:
>>> 
>>> Hello Yalin,
>>> 
>>> Sorry for missing you in Cc list.
>>> IIRC, mails to send your previous mail address(Yalin.Wang@xxxxxxxxxxxxxx)
>>> were returned.
>>> 
>>> You added comment bottom line so I'm not sure what PageDirty you meant.
>>> 
>>>> it is wrong here if you only check PageDirty() to decide if the page is freezable or not .
>>>> The Anon page are shared by multiple process, _mapcount > 1 ,
>>>> so you must check all pt_dirty bit during page_referenced() function,
>>>> see this mail thread:
>>>> http://ns1.ske-art.com/lists/kernel/msg1934021.html
>>> 
>>> If one of pte among process sharing the page was dirty, the dirtiness should
>>> be propagated from pte to PG_dirty by try_to_unmap_one.
>>> IOW, if the page doesn't have PG_dirty flag, it means all of process did
>>> MADV_FREE.
>>> 
>>> Am I missing something from you question?
>>> If so, could you show exact scenario I am missing?
>>> 
>>> Thanks for the interest.
>> oh, yeah , that is right , i miss that , pte_dirty will propagate to PG_dirty ,
>> so that is correct .
>> Generic to say this patch move set_page_dirty() from add_to_swap() to 
>> try_to_unmap(), i think can change a little about this patch:
>> 
>> @@ -1476,6 +1446,8 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
>> 				ret = SWAP_FAIL;
>> 				goto out_unmap;
>> 			}
>> +			if (!PageDirty(page))
>> +				SetPageDirty(page);
>> 			if (list_empty(&mm->mmlist)) {
>> 				spin_lock(&mmlist_lock);
>> 				if (list_empty(&mm->mmlist))
>> 
>> i think this 2 lines can be removed ,
>> since  pte_dirty have propagated to set_page_dirty() , we donât need this line here ,
>> otherwise you will always dirty a AnonPage, even it is clean,
>> then we will page out this clean page to swap partition one more , this is not needed.
>> am i understanding correctly ?
> 
> Your understanding is correct.
> I will fix it in next spin.
> 
>> 
>> By the way, please change my mail address to yalin.wang2010@xxxxxxxxx in CC list .
>> Thanks a lot. :) 
> 
> Thanks for the review!

i have a look at the old mail list , i recall the scenario that multiple processes share a AnonPage 
special case :

for example Process A have a AnonPage map like this:
	! pte_dirty() && PageDirty()==1   (this is possible after read fault happened on swap entry, and try_to_free_swap() succeed.)
Process A  do a fork() , New process is called B .
Then A  syscall(MADV_FREE) on the page .
At this time, page table like this:

A  ! pte_dirty() && PageDirty() == 0  && PageSwapCache() == 0

B ! pte_dirty() && PageDirty() == 0  && PageSwapCache() == 0

This means this page is freeable , and can be freed during page reclaim.
This is not fair for Process B . Since B donât call syscall(MADV_FREE) ,
its page should not be discard .  Will cause some strange behaviour if happened .

This is discussed by 
http://www.serverphorums.com/read.php?12,1220840
but i donât know why the patch is not merged .

Thanks 












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