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

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Date:   Tue, 2 Feb 2021 13:38:43 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] userfaultfd: hugetlbfs: only compile UFFD helpers if
 config enabled

On 2/2/21 12:31 PM, 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(+)

When you move this back into the "userfaultfd: add minor fault handling"
series, feel free to add:

Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>

Thanks,
-- 
Mike Kravetz