linux-kernel - Re: [PATCH v3 2/3] userfaultfd: UFFDIO

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Message-ID: <ZTa9Y++/PCV7HRoM@x1n>
Date:   Mon, 23 Oct 2023 14:37:23 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, viro@...iv.linux.org.uk,
        brauner@...nel.org, shuah@...nel.org, aarcange@...hat.com,
        lokeshgidra@...gle.com, david@...hat.com, hughd@...gle.com,
        mhocko@...e.com, axelrasmussen@...gle.com, rppt@...nel.org,
        willy@...radead.org, Liam.Howlett@...cle.com, jannh@...gle.com,
        zhangpeng362@...wei.com, bgeffon@...gle.com,
        kaleshsingh@...gle.com, ngeoffray@...gle.com, jdduke@...gle.com,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v3 2/3] userfaultfd: UFFDIO_MOVE uABI

On Mon, Oct 23, 2023 at 10:43:49AM -0700, Suren Baghdasaryan wrote:
> > Maybe we should follow what it does with mremap()?  Then your current code
> > is fine.  Maybe that's the better start.
> 
> I think that was the original intention, basically treating remapping
> as a write operation. Maybe I should add a comment here to make it
> more clear?

Please avoid mention "emulate as a write" - this is not a write, e.g., we
move a swap entry over without faulting in the page.  We also keep the page
states, e.g. on hotness.  A write will change all of that.

Now rethinking with the recently merged WP_ASYNC: we ignore uffd-wp, which
means dirty from uffd-wp async tracking POV, that matches with soft-dirty
always set.  Looks all good.

Perhaps something like "Follow mremap() behavior; ignore uffd-wp for now"
should work?

-- 
Peter Xu