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Date: Thu, 11 Jul 2024 05:59:56 +0200
From: Christoph Hellwig <hch@lst.de>
To: "Darrick J. Wong" <djwong@kernel.org>
Cc: John Garry <john.g.garry@oracle.com>, chandan.babu@oracle.com,
	dchinner@redhat.com, hch@lst.de, viro@zeniv.linux.org.uk,
	brauner@kernel.org, jack@suse.cz, linux-xfs@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	catherine.hoang@oracle.com, martin.petersen@oracle.com
Subject: Re: [PATCH v2 07/13] xfs: Introduce FORCEALIGN inode flag
Message-ID: <20240711035956.GA2556@lst.de>
References: <20240705162450.3481169-1-john.g.garry@oracle.com> <20240705162450.3481169-8-john.g.garry@oracle.com> <20240711025958.GJ612460@frogsfrogsfrogs>
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On Wed, Jul 10, 2024 at 07:59:58PM -0700, Darrick J. Wong wrote:
> Hmm.  If we don't support reflink + forcealign ATM, then shouldn't the
> superblock verifier or xfs_fs_fill_super fail the mount so that old
> kernels won't abruptly emit EFSCORRUPTED errors if a future kernel adds
> support for forcealign'd cow and starts writing out files with both
> iflags set?

Yes.

> That said, if the bs>ps patchset lands, then I think forcealign cow is
> a simple matter of setting the min folio order to the forcealign size
> and making sure that we always write out entire folios if any of the
> blocks cached by the folio is shared.  Direct writes to forcealigned
> shared files probably has to be aligned to the forcealign size or fall
> back to buffered writes for cow.

It has all the same problems as rtexsize > 1 + reflink, and suppoting
it will require raiding your patch stack.  Or better just wait until
we've got all that in now that we're actively working on it.