linux-kernel - Re: [PATCH v4 4/4] mm: userfaultfd: add UFFDIO_CONTINUE_MODE

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Date:   Thu, 9 Mar 2023 11:10:42 +0200
From:   Mike Rapoport <rppt@...nel.org>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>, Jan Kara <jack@...e.cz>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Nadav Amit <namit@...are.com>, Peter Xu <peterx@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        James Houghton <jthoughton@...gle.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v4 4/4] mm: userfaultfd: add UFFDIO_CONTINUE_MODE_WP to
 install WP PTEs

On Wed, Mar 08, 2023 at 02:19:32PM -0800, Axel Rasmussen wrote:
> UFFDIO_COPY already has UFFDIO_COPY_MODE_WP, so when installing a new
> PTE to resolve a missing fault, one can install a write-protected one.
> This is useful when using UFFDIO_REGISTER_MODE_{MISSING,WP} in
> combination.
> 
> This was motivated by testing HugeTLB HGM [1], and in particular its
> interaction with userfaultfd features. Existing userfaultfd code
> supports using WP and MINOR modes together (i.e. you can register an
> area with both enabled), but without this CONTINUE flag the combination
> is in practice unusable.
> 
> So, add an analogous UFFDIO_CONTINUE_MODE_WP, which does the same thing
> as UFFDIO_COPY_MODE_WP, but for *minor* faults.
> 
> Update the selftest to do some very basic exercising of the new flag.
> 
> [1]: https://patchwork.kernel.org/project/linux-mm/cover/20230218002819.1486479-1-jthoughton@google.com/
> 
> Acked-by: Peter Xu <peterx@...hat.com>
> Signed-off-by: Axel Rasmussen <axelrasmussen@...gle.com>

Acked-by: Mike Rapoport (IBM) <rppt@...nel.org>

> ---
>  fs/userfaultfd.c                         | 8 ++++++--
>  include/linux/userfaultfd_k.h            | 3 ++-
>  include/uapi/linux/userfaultfd.h         | 7 +++++++
>  mm/userfaultfd.c                         | 5 +++--
>  tools/testing/selftests/mm/userfaultfd.c | 4 ++++
>  5 files changed, 22 insertions(+), 5 deletions(-)
> 
> diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> index 005e5e306266..14059a0861bf 100644
> --- a/include/uapi/linux/userfaultfd.h
> +++ b/include/uapi/linux/userfaultfd.h
> @@ -297,6 +297,13 @@ struct uffdio_writeprotect {
>  struct uffdio_continue {
>  	struct uffdio_range range;
>  #define UFFDIO_CONTINUE_MODE_DONTWAKE		((__u64)1<<0)
> +	/*
> +	 * UFFDIO_CONTINUE_MODE_WP will map the page write protected on
> +	 * the fly.  UFFDIO_CONTINUE_MODE_WP is available only if the
> +	 * write protected ioctl is implemented for the range
> +	 * according to the uffdio_register.ioctls.
> +	 */
> +#define UFFDIO_CONTINUE_MODE_WP			((__u64)1<<1)

Please add the description of the new flag to Documentation/ and to the
userfaultfd man pages.

>  	__u64 mode;
>  
>  	/*

-- 
Sincerely yours,
Mike.