From: Linus Walleij <linus.walleij@linaro.org>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: "Suzuki K. Poulose" <suzuki.poulose@arm.com>,
"Theodore Ts'o" <tytso@mit.edu>,
Andreas Dilger <adilger.kernel@dilger.ca>,
Ext4 Developers List <linux-ext4@vger.kernel.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
Linux API <linux-api@vger.kernel.org>,
QEMU Developers <qemu-devel@nongnu.org>,
Florian Weimer <fw@deneb.enyo.de>,
Andy Lutomirski <luto@kernel.org>,
stable <stable@vger.kernel.org>
Subject: Re: [PATCH] ext4: Give 32bit personalities 32bit hashes
Date: Thu, 19 Mar 2020 23:23:33 +0100 [thread overview]
Message-ID: <CACRpkdYuZgZUznVxt1AHCSJa_GAXy8N0SduE5OrjDnE1s_L7Zg@mail.gmail.com> (raw)
In-Reply-To: <CAFEAcA-gdwi=KSW6LqVdEJWSo9VEL5abYQs9LoHd4mKE_-h=Aw@mail.gmail.com>
On Thu, Mar 19, 2020 at 4:25 PM Peter Maydell <peter.maydell@linaro.org> wrote:
> On Thu, 19 Mar 2020 at 15:13, Linus Walleij <linus.walleij@linaro.org> wrote:
> > On Tue, Mar 17, 2020 at 12:58 PM Peter Maydell <peter.maydell@linaro.org> wrote:
> > > What in particular does this personality setting affect?
> > > My copy of the personality(2) manpage just says:
> > >
> > > PER_LINUX32 (since Linux 2.2)
> > > [To be documented.]
> > >
> > > which isn't very informative.
> >
> > It's not a POSIX thing (not part of the Single Unix Specification)
> > so as with most Linux things it has some fuzzy semantics
> > defined by the community...
> >
> > I usually just go to the source.
>
> If we're going to decide that this is the way to say
> "give me 32-bit semantics" we need to actually document
> that and define in at least broad terms what we mean
> by it, so that when new things are added that might or
> might not check against the setting there is a reference
> defining whether they should or not, and so that
> userspace knows what it's opting into by setting the flag.
> The kernel loves undocumented APIs but userspace
> consumers of them are not so enamoured :-)
OK I guess we can at least take this opportunity to add
some kerneldoc to the include file.
> As a concrete example, should "give me 32-bit semantics
> via PER_LINUX32" mean "mmap should always return addresses
> within 4GB" ? That would seem like it would make sense --
Incidentally that thing in particular has its own personality
flag (personalities are additive, it's a bit schizophrenic)
so PER_LINUX_32BIT is defined as:
PER_LINUX_32BIT = 0x0000 | ADDR_LIMIT_32BIT,
and that is specifically for limiting the address space to
32bit.
There is also PER_LINUX32_3GB for a 3GB lowmem
limit.
Since the personality is kind of additive, if
we want a flag *specifically* for indicating that we want
32bit hashes from the file system, there are bits left so we
can provide that.
Is this what we want to do? I just think we shouldn't
decide on that lightly as we will be using up personality
bug bits, but sometimes you have to use them.
PER_LINUX32 as it stands means 32bit personality
but very specifically does not include memory range
limitations since that has its own flags.
Yours,
Linus Walleij
next prev parent reply other threads:[~2020-03-19 22:23 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-17 11:31 [PATCH] ext4: Give 32bit personalities 32bit hashes Linus Walleij
2020-03-17 11:52 ` Florian Weimer
2020-03-17 12:38 ` Linus Walleij
2020-03-17 11:58 ` Peter Maydell
2020-03-19 15:13 ` Linus Walleij
2020-03-19 15:25 ` Peter Maydell
2020-03-19 22:23 ` Linus Walleij [this message]
2020-03-24 2:34 ` Theodore Y. Ts'o
2020-03-24 9:29 ` Peter Maydell
2020-03-24 18:47 ` Theodore Y. Ts'o
2020-03-24 21:17 ` Linus Walleij
2020-03-17 22:28 ` Andreas Dilger
2020-03-19 15:18 ` Linus Walleij
2020-03-17 22:30 ` Sasha Levin
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