linux-kernel - Re: [PATCH v5 4/9] xfs: convert delayed extents to unwritten when zeroing post eof blocks

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Date: Thu, 25 Apr 2024 23:33:01 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Zhang Yi <yi.zhang@...weicloud.com>
Cc: "Darrick J. Wong" <djwong@...nel.org>, linux-xfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	hch@...radead.org, brauner@...nel.org, david@...morbit.com,
	chandanbabu@...nel.org, tytso@....edu, jack@...e.cz,
	yi.zhang@...wei.com, chengzhihao1@...wei.com, yukuai3@...wei.com
Subject: Re: [PATCH v5 4/9] xfs: convert delayed extents to unwritten when
 zeroing post eof blocks

On Fri, Apr 26, 2024 at 02:24:19PM +0800, Zhang Yi wrote:
> Yeah, it looks more reasonable. But from the original scene, the
> xfs_bmap_extsize_align() aligned the new extent that added to the cow fork
> could overlaps the unreflinked range, IIUC, I guess that spare range is
> useless exactly, is there any situation that would use it?

I've just started staring at this (again) half an hour ago, and I fail
to understand the (pre-existing) logic in xfs_reflink_zero_posteof.

We obviously need to ensure data between i_size and the end of the
block that i_size sits in is zeroed (but IIRC we already do that
in write and truncate anyway).  But what is the point of zeroing
any speculative preallocation beyond the last block that actually
contains data?  Just truncating the preallocation and freeing
the delalloc and unwritten blocks seems like it would be way
more efficient.