Re: [PATCH v3 2/3] userfaultfd: UFFDIO_MOVE uABI

From: Suren Baghdasaryan <surenb@google.com>
To: Peter Xu <peterx@redhat.com>
Cc: David Hildenbrand <david@redhat.com>,
	Lokesh Gidra <lokeshgidra@google.com>,
	akpm@linux-foundation.org,  viro@zeniv.linux.org.uk,
	brauner@kernel.org, shuah@kernel.org,  aarcange@redhat.com,
	hughd@google.com, mhocko@suse.com,  axelrasmussen@google.com,
	rppt@kernel.org, willy@infradead.org,  Liam.Howlett@oracle.com,
	jannh@google.com, zhangpeng362@huawei.com,  bgeffon@google.com,
	kaleshsingh@google.com, ngeoffray@google.com,  jdduke@google.com,
	linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
	 linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	 kernel-team@android.com
Subject: Re: [PATCH v3 2/3] userfaultfd: UFFDIO_MOVE uABI
Date: Thu, 19 Oct 2023 13:02:39 -0700	[thread overview]
Message-ID: <CAJuCfpEVgLtc3iS_huxbr86bNwEix+M4iEqWeQYUbsP6KcxfQQ@mail.gmail.com> (raw)
In-Reply-To: <ZTGJHesvkV84c+l6@x1n>

On Thu, Oct 19, 2023 at 12:53 PM Peter Xu <peterx@redhat.com> wrote:
>
> On Thu, Oct 19, 2023 at 05:41:01PM +0200, David Hildenbrand wrote:
> > That's not my main point. It can easily become a maintenance burden without
> > any real use cases yet that we are willing to support.
>
> That's why I requested a few times that we can discuss the complexity of
> cross-mm support already here, and I'm all ears if I missed something on
> the "maintenance burden" part..
>
> I started by listing what I think might be different, and we can easily
> speedup single-mm with things like "if (ctx->mm != mm)" checks with
> e.g. memcg, just like what this patch already did with pgtable depositions.
>
> We keep saying "maintenance burden" but we refuse to discuss what is that..
>
> I'll leave that to Suren and Lokesh to decide.  For me the worst case is
> one more flag which might be confusing, which is not the end of the world..
> Suren, you may need to work more thoroughly to remove cross-mm implications
> if so, just like when renaming REMAP to MOVE.

Hi Folks,
Sorry, I'm just catching up on all the comments in this thread after a
week-long absence. Will be addressing other questions separately but
for cross-mm one, I think the best way forward would be for me to
split this patch into two with the second one adding cross-mm support.
That will clearly show how much additional code that requires and will
make it easier for us to decide whether to support it or not.
TBH, I don't see the need for an additional flag even if the initial
version will be merged without cross-mm support. Once it's added the
manpage can mention that starting with a specific Linux version
cross-mm is supported, no?
Also from my quick read, it sounds like we want to prevent movements
of pinned pages regardless of cross-mm support. Is my understanding
correct?
Thanks,
Suren.

>
> Thanks,
>
> --
> Peter Xu
>