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

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Date:   Sun, 7 Apr 2019 13:10:55 -0500
From:   Eric Sandeen <sandeen@...deen.net>
To:     Theodore Ts'o <tytso@....edu>,
        "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     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 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...

-Eric
 
> That's not guaranteed to work for all file systems, of course.  So the
> right answer may be to define a new IOCTL which causes all file system
> to do whatever log truncation is needed so that grub will do the right
> thing.
> 
> 					- Ted
>