Re: [PATCH 1/5] mm/vmscan: put the redirtied MADV_FREE pages back to anonymous LRU list

[Miaohe Lin]

On 2021/7/13 21:34, Matthew Wilcox wrote:
> On Tue, Jul 13, 2021 at 09:13:51PM +0800, Miaohe Lin wrote:
>>>> When the MADV_FREE pages are redirtied before they could be reclaimed, the pages
>>>> should be put back to anonymous LRU list by setting SwapBacked flag, thus the
>>>> pages will be reclaimed in normal swapout way.
>>>
>>> Agreed. But the question is why this needs an explicit handling here
>>> when we already do handle this case when trying to unmap the page.
>>
>> This makes me think more. It seems even the page_ref_freeze call is guaranteed to
>> success as no one can grab the page refcnt after the page is successfully unmapped.
> 
> NO!  This is wrong.  Every page can have its refcount speculatively raised
> (and then lowered).  The two prime candidates for this are lockless GUP
> and page cache lookups, but there can be others too.
> 

Many thanks for pointing this out. My overlook! Sorry!
So, it seems lockless GUP can redirty the MADV_FREE page. But is it ok to just release
a redirtied MADV_FREE pages? Because we hold the last reference here and the page will
be freed anyway...

> .
>