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Date: Mon, 5 Feb 2024 11:32:49 -0500
From: "Theodore Ts'o" <tytso@mit.edu>
To: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Cc: linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org, jack@suse.cz,
        ritesh.list@gmail.com, john.g.garry@oracle.com, djwong@kernel.org
Subject: Re: Running out of inode flags in ext4
Message-ID: <20240205163249.GE119530@mit.edu>
References: <ZcCztkt8KtMrsvPp@li-bb2b2a4c-3307-11b2-a85c-8fa5c3a69313.ibm.com>
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On Mon, Feb 05, 2024 at 03:38:54PM +0530, Ojaswin Mujoo wrote:
> Hi folks,
> 
> I'm trying to rework the ext4 atomic write patchset so we have similar
> semantics as discussed here [1], which would look something like:
> 
> 1. we call FS_IOC_FSSETXATTR to enable atomic write on inode
> 
> 2. In the setxattr path, we need to mark the inode with
> atomic_write_enabled. XFS does it via an on disk inode flag which is
> straightforward enough 
> 
> However, on ext4 we are almost out of 32 bits of inode flags and I don't
> think it's possible to add any flags2 field or something (iiuc, ondisk
> indoe doesn't have scope for expansion).

We still have some unused flags.  For example,
0x01000000. 0x04000000. and 0x08000000 are still unused.  We are
starting to get close to full, so we need to be a bit careful since it
is very much a limited resource.  But we're not yet at the point where
we need to worry about trying to reuse flags like EXT4_EOFBLOCKS_FL.

Cheers,

							- Ted