linux-ext4 - Re: [PATCH] bootfs: simple bootloader filesystem

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Date:   Mon, 8 Apr 2019 05:28:23 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     Eric Sandeen <sandeen@...deen.net>, Theodore Ts'o <tytso@....edu>,
        Dave Chinner <david@...morbit.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        xfs <linux-xfs@...r.kernel.org>
Subject: Re: [PATCH] bootfs: simple bootloader filesystem

On Apr 7, 2019, at 2:13 PM, Darrick J. Wong <darrick.wong@...cle.com> wrote:
> 
> On Sun, Apr 07, 2019 at 01:10:55PM -0500, Eric Sandeen wrote:
>> On 4/6/19 6:27 PM, Theodore Ts'o wrote:
>>> On Mon, Apr 01, 2019 at 09:55:19PM -0700, Darrick J. Wong wrote:
>>>> 
>>>> When Ted is done laughing, I really would like to consider something
>>>> like this to solve the problem of grub-style bootloaders requiring a
>>>> lease on the blocks underneath a file with a term exceeding that of the
>>>> running kernel.
>>>> 
>>>> We can probably skip the harsh synchronous writes in favor of fsync on
>>>> close, but we would need to keep the critical component of checkpointing
>>>> the journal on fsync and syncfs.
>>> 
>>> At least for ext4, we don't need to add anything new, since FIFREEZE
>>> force a journal checkpoint.  So we could try to get a patch into grub
>>> which causes update_grub to open each kernel that it finds, and calls
>>> fsync(2) on it, and then for all file systems where it finds a kernel,
>>> it can call FIFREEZE and FITHAW on it, and that would be that.
>> 
>> Certain operating systems have hacked this in.  My concern would be when
>> /boot is on / ... calling FIFREEZE on the root fs would most likely be
>> a bad thing.  Certain operating systems avoid calling FIFREEZE for
>> /boot-on-root.  ;)
>> 
>> Doing it for a standalone /boot seems like a reasonable (if hacky)
>> workaround as long as we lack a more targeted quiesce interface...
> 
> The other problem we noticed is that neither the grub scripts nor the
> rpm package scripts bother to call fsync on the files they write (or
> sync after they're done to mop up after everyone else), so I figured as
> long as I'm ("jokingly") working around it all in kernel space, why not
> just go all the way? :P
> 
> Ok, I'll go work on an ioctl or something.

If Grub isn't even bothering to call fsync() on a file, what is the chance
that they would call a special ioctl on the file?

What about doing "chattr +S /boot" so that all file IO in this directory is
done synchronously, which would work even if /boot is not on a separate
filesystem?  The "+S" flag is inherited by new files created in the directory.

Cheers, Andreas






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