linux-kernel - Re: [PATCH v2 3/5] mm: userfaultfd: combine 'mode' and 'wp

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Date:   Thu, 23 Feb 2023 18:51:01 +0000
From:   Nadav Amit <namit@...are.com>
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>,
        Mike Rapoport <rppt@...nel.org>,
        Muchun Song <muchun.song@...ux.dev>,
        Peter Xu <peterx@...hat.com>, Shuah Khan <shuah@...nel.org>,
        James Houghton <jthoughton@...gle.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH v2 3/5] mm: userfaultfd: combine 'mode' and 'wp_copy'
 arguments



> On Feb 22, 2023, at 4:57 PM, Axel Rasmussen <axelrasmussen@...gle.com> wrote:
> 
> Many userfaultfd ioctl functions take both a 'mode' and a 'wp_copy'
> argument. In future commits we plan to plumb the flags through to more
> places, so we'd be proliferating the very long argument list even
> further.
> 
> Let's take the time to simplify the argument list. Combine the two
> arguments into one - and generalize, so when we add more flags in the
> future, it doesn't imply more function arguments.
> 
> Since the modes (copy, zeropage, continue) are mutually exclusive, store
> them as an integer value (0, 1, 2) in the low bits. Place combine-able
> flag bits in the high bits.
> 
> Signed-off-by: Axel Rasmussen <axelrasmussen@...gle.com>

Hi Axel,

I sent a patch a long time ago called “userfaultfd: introduce uffd_flags”.
For some reason it does not appear on lore, but you were a recipient.

It was pretty similar, but one thing that I preferred in my version is
that it defined a different type to avoid confusion. Since “flags” is
a very generic name, perhaps you’d like to adapt this approach.