From: Peter Xu <peterx@redhat.com>
To: Axel Rasmussen <axelrasmussen@google.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
Andrew Morton <akpm@linux-foundation.org>,
Hugh Dickins <hughd@google.com>, Jan Kara <jack@suse.cz>,
"Liam R. Howlett" <Liam.Howlett@oracle.com>,
Matthew Wilcox <willy@infradead.org>,
Mike Kravetz <mike.kravetz@oracle.com>,
Mike Rapoport <rppt@kernel.org>,
Muchun Song <muchun.song@linux.dev>,
Nadav Amit <namit@vmware.com>, Shuah Khan <shuah@kernel.org>,
James Houghton <jthoughton@google.com>,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-mm@kvack.org, linux-kselftest@vger.kernel.org
Subject: Re: [PATCH v3 3/5] mm: userfaultfd: combine 'mode' and 'wp_copy' arguments
Date: Wed, 8 Mar 2023 10:17:25 -0500 [thread overview]
Message-ID: <ZAinBTbXu4ZriATC@x1n> (raw)
In-Reply-To: <CAJHvVciQWctUoZtrPga-fhgBf2dtc+6ypwE3FYe8ApQWpQyL0Q@mail.gmail.com>
On Tue, Mar 07, 2023 at 03:27:17PM -0800, Axel Rasmussen wrote:
> >
> > >
> > > +#define MFILL_ATOMIC_MODE_BITS (const_ilog2(NR_MFILL_ATOMIC_MODES - 1) + 1)
> >
> > Here IIUC it should be "const_ilog2(NR_MFILL_ATOMIC_MODES) + 1", but
> > maybe.. we don't bother and define every bit explicitly?
>
> If my reading of const_ilog2's definition is correct, then:
>
> const_ilog2(4) = 2
> const_ilog2(3) = 1
> const_ilog2(2) = 1
>
> For either 3 or 4 modes, we need 2 bits to represent them (0, 1, 2,
> 3), i.e. we want MFILL_ATOMIC_MODE_BITS = 2. I think this is correct
> as is, because const_ilog2(4 - 1) + 1 = 2, and const_ilog2(3 - 1) + 1
> = 2.
>
> In other words, I think const_ilog2 is defined as floor(log2()),
> whereas what we want is ceil(log2()).
You're right.
>
> The benefit of doing this vs. just doing defines with fixed values is,
> if we ever added a new mode, we wouldn't have to do bit twiddling and
> update the mask, flag bits, etc. - it would happen "automatically". I
> prefer it this way, but I agree it is a matter of opinion / taste. :)
> If you or others feel strongly this is overcomplicated, I can take the
> other approach.
I don't know what this will look like at last. The thing is if you plan to
define MFILL_ATOMIC_* with __bitwise I think it'll stop working with any
calculations upon it.
I don't worry on growing modes, as I don't expect it to happen a lot.
No strong opinion here, as long as sparse won't complain.
Thanks,
--
Peter Xu
next prev parent reply other threads:[~2023-03-08 15:18 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-06 22:50 [PATCH v3 0/5] mm: userfaultfd: refactor and add UFFDIO_CONTINUE_MODE_WP Axel Rasmussen
2023-03-06 22:50 ` [PATCH v3 1/5] mm: userfaultfd: rename functions for clarity + consistency Axel Rasmussen
2023-03-07 1:03 ` Peter Xu
2023-03-06 22:50 ` [PATCH v3 2/5] mm: userfaultfd: don't pass around both mm and vma Axel Rasmussen
2023-03-07 1:03 ` Peter Xu
2023-03-07 1:44 ` Nadav Amit
2023-03-08 15:08 ` Peter Xu
2023-03-06 22:50 ` [PATCH v3 3/5] mm: userfaultfd: combine 'mode' and 'wp_copy' arguments Axel Rasmussen
2023-03-07 1:00 ` Peter Xu
2023-03-07 23:27 ` Axel Rasmussen
2023-03-08 15:17 ` Peter Xu [this message]
2023-03-07 1:54 ` Nadav Amit
2023-03-06 22:50 ` [PATCH v3 4/5] mm: userfaultfd: don't separate addr + len arguments Axel Rasmussen
2023-03-07 1:19 ` Peter Xu
2023-03-07 1:29 ` Nadav Amit
2023-03-07 18:52 ` Axel Rasmussen
2023-03-08 9:51 ` kernel test robot
2023-03-08 18:48 ` Axel Rasmussen
2023-03-06 22:50 ` [PATCH v3 5/5] mm: userfaultfd: add UFFDIO_CONTINUE_MODE_WP to install WP PTEs Axel Rasmussen
2023-03-07 1:23 ` Peter Xu
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