linux-kernel - Re: [PATCH] userfaultfd: hugetlbfs: only compile UFFD helpers if config enabled

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Date:   Tue, 2 Feb 2021 15:24:56 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
cc:     Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Peter Xu <peterx@...hat.com>, Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] userfaultfd: hugetlbfs: only compile UFFD helpers if
 config enabled

On Tue, 2 Feb 2021, Axel Rasmussen wrote:
> On Tue, Feb 2, 2021 at 1:03 PM Hugh Dickins <hughd@...gle.com> wrote:
> > On Tue, 2 Feb 2021, Axel Rasmussen wrote:
> >
> > > For background, mm/userfaultfd.c provides a general mcopy_atomic
> > > implementation. But some types of memory (e.g., hugetlb and shmem) need
> > > a slightly different implementation, so they provide their own helpers
> > > for this. In other words, userfaultfd is the only caller of this
> > > function.
> > >
> > > This patch achieves two things:
> > >
> > > 1. Don't spend time compiling code which will end up never being
> > > referenced anyway (a small build time optimization).
> > >
> > > 2. In future patches (e.g. [1]), we plan to extend the signature of
> > > these helpers with UFFD-specific state (e.g., enums or structs defined
> > > conditionally in userfaultfd_k.h). Once this happens, this patch will be
> > > needed to avoid build errors (or, we'd need to define more UFFD-only
> > > stuff unconditionally, which seems messier to me).
> > >
> > > Peter Xu suggested this be sent as a standalone patch, in the mailing
> > > list discussion for [1].
> > >
> > > [1] https://patchwork.kernel.org/project/linux-mm/list/?series=424091
> > >
> > > Signed-off-by: Axel Rasmussen <axelrasmussen@...gle.com>
> > > ---
> > >  include/linux/hugetlb.h | 4 ++++
> > >  mm/hugetlb.c            | 2 ++
> > >  2 files changed, 6 insertions(+)
> >
> > Hi Axel, please also do the same to mm/shmem.c (perhaps you missed
> > it because I did that long ago to our internal copy of mm/shmem.c).
> 
> I had been largely ignoring shmem up to this point because my minor
> fault handling series doesn't (yet) deal with it. But, I'll need to do
> this later when I support shmem anyway, so happy to add it here.

Oh, if this patch is going into a hugetlbfs series, skip mm/shmem.c for
now (or keep it in, whichever's easiest for you): I caught sight of the
"(e.g., hugetlb and shmem)" in the commit message above, and thought
you had inadvertently missed out the shmem part - but now see that
the patch title does say "userfaultfd: hugetlbfs:".

Hugh