linux-kernel - Re: [PATCH RFC] mm: userfaultfd: correct dirty flags set for both present and swap pte

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Message-ID: <CAGsJ_4y=9_NYkQDPVp5dcsr70EE=fBOmwpoRM623uwB-+7Q1Ug@mail.gmail.com>
Date: Fri, 9 May 2025 09:36:11 +1200
From: Barry Song <21cnbao@...il.com>
To: Lokesh Gidra <lokeshgidra@...gle.com>
Cc: Peter Xu <peterx@...hat.com>, akpm@...ux-foundation.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Barry Song <v-songbaohua@...o.com>, 
	David Hildenbrand <david@...hat.com>, Suren Baghdasaryan <surenb@...gle.com>, 
	Kalesh Singh <kaleshsingh@...gle.com>
Subject: Re: [PATCH RFC] mm: userfaultfd: correct dirty flags set for both
 present and swap pte

On Fri, May 9, 2025 at 3:27 AM Lokesh Gidra <lokeshgidra@...gle.com> wrote:
>
> Thanks Barry for fixing this.
>
> On Thu, May 8, 2025 at 8:24 AM Peter Xu <peterx@...hat.com> wrote:
> >
> > On Thu, May 08, 2025 at 09:07:35PM +1200, Barry Song wrote:
> > > From: Barry Song <v-songbaohua@...o.com>
> > >
> > > As David pointed out, what truly matters for mremap and userfaultfd
> > > move operations is the soft dirty bit. The current comment and
> > > implementation—which always sets the dirty bit for present PTEs
> > > and fails to set the soft dirty bit for swap PTEs—are incorrect.
>
> Can you please briefly describe the consequences of not setting the
> soft-dirty bit? I'm wondering if it needs to be backported as a fix?

As I understand it, this could break features like Checkpoint-Restore
in Userspace (CRIU), which relies on tracking memory changes to create
incremental dumps. While Android may not currently have a real-world
use case for this, it would still be beneficial to backport the fix in
a general way.

>
> > > This patch updates the behavior to correctly set the soft dirty bit
> > > for both present and swap PTEs in accordance with mremap.
> > >
> > > Reported-by: David Hildenbrand <david@...hat.com>
> > > Closes: https://lore.kernel.org/linux-mm/02f14ee1-923f-47e3-a994-4950afb9afcc@redhat.com/
> > > Cc: Peter Xu <peterx@...hat.com>
> > > Cc: Suren Baghdasaryan <surenb@...gle.com>
> > > Cc: Lokesh Gidra <lokeshgidra@...gle.com>
> > > Signed-off-by: Barry Song <v-songbaohua@...o.com>
> >
> > Acked-by: Peter Xu <peterx@...hat.com>
> >
> > --
> > Peter Xu

Thanks
Barry