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

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Message-ID: <CAJuCfpEVgLtc3iS_huxbr86bNwEix+M4iEqWeQYUbsP6KcxfQQ@mail.gmail.com>
Date:   Thu, 19 Oct 2023 13:02:39 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Lokesh Gidra <lokeshgidra@...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 12:53 PM Peter Xu <peterx@...hat.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
>