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

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Date:   Mon, 7 Feb 2022 23:05:03 -0300
From:   Pedro Gomes <pedrodemargomes@...il.com>
To:     David Hildenbrand <david@...hat.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 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.


-- 
Atenciosamente,
Pedro Demarchi Gomes.