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Date:   Tue, 8 Feb 2022 10:01:32 +0100
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Subject: Re: [PATCH] mm/damon: Add option to monitor only writes
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To:     Pedro Gomes <pedrodemargomes@gmail.com>
Cc:     SeongJae Park <sj@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, John Hubbard <jhubbard@nvidia.com>
References: <20220203131237.298090-1-pedrodemargomes@gmail.com>
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <CACpbsfapGSbno_n7ZJYJ2E4KkqWdWe_Wz8PuECBa3PZBp2Xnxw@mail.gmail.com>
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On 08.02.22 03:05, Pedro Gomes wrote:
> On Mon, Feb 7, 2022 at 7:32 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> Just like clearrefs, this can race against GUP-fast to detect pinned
>> pages. And just like clearrefs, we're not handling PMDs properly. And
>> just like anything that write-protects random anon pages right now, this
>> does not consider O_DIRECT as is.
>>
>> Fortunately, there are not too many users of clearreefs/softdirty
>> tracking out there (my search a while ago returned no open source
>> users). My assumption is that your feature might see more widespread use.
>>
>> Adding more random write protection until we fixed the COW issues [1]
>> really makes my stomach hurt on a Monday morning.
> 
> I was not aware of these issues.
> 
>> Please, let's defer any more features that rely on write-protecting
>> random anon pages until we have ways in place to not corrupt random user
>> space.
>>
>> That is:
>> 1) Teaching the COW logic to not copy pages that are pinned -- I'm
>> working on that.
>> 2) Converting O_DIRECT to use FOLL_PIN instead of FOLL_GET. John is
>> working on that.
>>
>> So I'm not against this change. I'm against this change at this point in
>> time.
> 
> I agree. I will wait until the COW problems are solved to send this patch.
> 
> 

I'll put you on CC once I have something ready to at least handle pinned
pages (FOLL_PIN) as expected, to not get them accidentally COWed.

-- 
Thanks,

David / dhildenb