linux-kernel - Re: [PATCH] mm/damon: Add option to monitor only writes

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Date:   Tue, 8 Feb 2022 10:01:32 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Pedro Gomes <pedrodemargomes@...il.com>
Cc:     SeongJae Park <sj@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH] mm/damon: Add option to monitor only writes

On 08.02.22 03:05, Pedro Gomes wrote:
> On Mon, Feb 7, 2022 at 7:32 AM David Hildenbrand <david@...hat.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