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Subject: Re: [PATCH 1/5] mm/vmscan: put the redirtied MADV_FREE pages back to
 anonymous LRU list
To:     John Hubbard <jhubbard@nvidia.com>
CC:     Michal Hocko <mhocko@suse.com>, Yu Zhao <yuzhao@google.com>,
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        Matthew Wilcox <willy@infradead.org>
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From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <1e1ce97a-2a0c-d0a4-85ad-cf70a85d9146@huawei.com>
Date:   Fri, 16 Jul 2021 09:53:51 +0800
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On 2021/7/16 8:01, John Hubbard wrote:
> On 7/15/21 4:30 AM, Miaohe Lin wrote:
> ...
>>>>> 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...
>>>>
>>>> I don't see how lockless GUP can redirty the page.  It can grab the
>>>> refcount, thus making the refcount here two.  Then the call to freeze
>>>> here fails and the page stays on the list.  But the lockless GUP checks
>>>> the page is still in the page table (and discovers it isn't, so releases
>>>> the reference count).  Am I missing a path that lets lockless GUP dirty
>>>> the page?
>>>>
>>>
>>> If a device driver pins some pages using gup, and the device then uses dma
>>> to write to those pages, then you could get there. That story is part of the
>>> reasoning that led to creating pin_user_pages(), which btw does not yet
>>> fully solve that case.
>>
>> Many thanks for your explanation.
>> So the similar scenario that is clarified in the __remove_mapping() is possible:
> 
> I probably should have added that the scenario I was describing is broken even
> before any patches that you might apply here. I was just trying to ensure that
> the complete list of scenarios was known.
> 

Many thanks for doing this! :)

> 
> 
> thanks,