> On Oct 27, 2015, at 16:10, Minchan Kim <minc...@kernel.org> wrote:
>
> On Tue, Oct 27, 2015 at 03:39:16PM +0800, yalin wang wrote:
>>
>>> On Oct 27, 2015, at 15:09, Minchan Kim <minc...@kernel.org> wrote:
>>>
>>> Hello Yalin,
>>>
>>> Sorry for missing you in Cc list.
>>> IIRC, mails to send your previous mail address(yalin.w...@sonymobile.com)
>>> 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 don’t 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.wang2...@gmail.com 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 don’t 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 don’t know why the patch is not merged .
Thanks
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