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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 3 Mar 2011 15:36:27 -0800
Message-ID: <AANLkTimXhGRsuL3Z51msS_YEtsa4UZuN9XW8Xb+hu_KN@mail.gmail.com>
Subject: Re: [PATCH] Syscalls: reboot: Add options to the reboot syscall to
 remount filesystems ro
To: Mark Lord <kernel@teksavvy.com>
Cc: Ken Sumrall <ksumrall@android.com>, linux-kernel@vger.kernel.org,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@lst.de>,
        Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.cz>,
        Jens Axboe <axboe@kernel.dk>, Matthew Wilcox <matthew@wil.cx>,
        Eric Paris <eparis@redhat.com>, Dave Young <hidave.darkstar@gmail.com>,
        Jiri Slaby <jslaby@suse.cz>, James Morris <jmorris@namei.org>,
        linux-fsdevel@vger.kernel.org
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On Thu, Mar 3, 2011 at 10:28 AM, Mark Lord <kernel@teksavvy.com> wrote:
> On 11-03-03 12:45 PM, Linus Torvalds wrote:
>>
>> If you can change whatever user-land process that does the reboot
>> system call (and clearly you can, since you're adding new commands and
>> using those), then why the heck don't you just do the remount-ro from
>> that same user land?
>
> Is there a system call for emergency_remount_*() ?
> I've been writing to /proc/proc/sysrq-trigger to accomplish this.

So I don't know what this has to do with "emergency_remount()" - we're
talking about a regular controlled shutdown/reset. The fact that the
patch used the emergency_remount() code seems to be purely an
implementation issue, nothing more.

And while sysrq-trigger certainly works (when it's enabled, but that's
true of /proc too, of course), I do think it's a rather odd way of
solving the problem, when the simple "just remount read-only" is what
the code actually _wants_ to do.

But you're certainly right that it takes less code to open
/proc/sysrq-trigger and writing a single byte to it than it does to do
the straightforward "let's just do the normal mount thing".

                             Linus
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