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

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Message-ID: <ZTGJHesvkV84c+l6@x1n>
Date:   Thu, 19 Oct 2023 15:53:01 -0400
From:   Peter Xu <peterx@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Lokesh Gidra <lokeshgidra@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        akpm@...ux-foundation.org, viro@...iv.linux.org.uk,
        brauner@...nel.org, shuah@...nel.org, aarcange@...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 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.

Thanks,

-- 
Peter Xu