linux-ext4 - Re: [PATCH 2/2] ext4: add ioctl EXT4_FLUSH

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Date:   Tue, 30 Mar 2021 13:15:08 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     "Darrick J. Wong" <djwong@...nel.org>
Cc:     harshad shirwadkar <harshadshirwadkar@...il.com>,
        Leah Rumancik <leah.rumancik@...il.com>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 2/2] ext4: add ioctl EXT4_FLUSH_JOURNAL

On Tue, Mar 30, 2021 at 09:32:23AM -0700, Darrick J. Wong wrote:
> Why not make discarding the journal part of FITRIM then?

Unfortunately, the fstrim_range structure doesn't have a place for a
flags field, and FITRIM works by specifying a range of LBA's:

struct fstrim_range {
	__u64 start;
	__u64 len;
	__u64 minlen;
};

I suppose we could do something where some combination of start/len
means "also checkpoint and discard the journal", but that seems rather
kludgy.

> It occurred to me overnight that another way to look at this ioctl
> proposal is that it checkpoints the filesystem and has a flag to discard
> the journal blocks too.  Given that we're now only two days away from
> my traditional bootfs[1] drum-banging day, and there's real user
> demand[2] for bootloaders to be able to force a journal checkpoint,

How about if we have an ioctl which is "checkpoint journal", which can
be file system independent (e.g., defined in include/uapi/linux/fs.h)
which takes a u32 flags field, where we define a flag bit to mean
"also discard the unused part of the journal after the checkpoint"?

It seems that would also solve your bootfs() use case.

      	      	  	      	     - Ted