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Subject: Re: [PATCH 1/5] mm/vmscan: put the redirtied MADV_FREE pages back to
 anonymous LRU list
To:     Miaohe Lin <linmiaohe@huawei.com>,
        Matthew Wilcox <willy@infradead.org>
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From:   John Hubbard <jhubbard@nvidia.com>
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Date:   Thu, 15 Jul 2021 17:01:45 -0700
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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.


thanks,
-- 
John Hubbard
NVIDIA