linux-kernel - Re: [PATCH 1/5] mm/vmscan: put the redirtied MADV

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Date:   Thu, 15 Jul 2021 17:01:45 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Miaohe Lin <linmiaohe@...wei.com>,
        Matthew Wilcox <willy@...radead.org>
CC:     Michal Hocko <mhocko@...e.com>, Yu Zhao <yuzhao@...gle.com>,
        <akpm@...ux-foundation.org>, <hannes@...xchg.org>,
        <vbabka@...e.cz>, <axboe@...nel.dk>, <iamjoonsoo.kim@....com>,
        <alexs@...nel.org>, <apopple@...dia.com>, <minchan@...nel.org>,
        <david@...hat.com>, <shli@...com>, <hillf.zj@...baba-inc.com>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/5] mm/vmscan: put the redirtied MADV_FREE pages back to
 anonymous LRU list

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