linux-kernel - Re: [RFC PATCH v4 8/8] xfs: improve truncate on a realtime inode with huge extsize

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Date: Fri, 31 May 2024 07:15:34 -0700
From: Christoph Hellwig <hch@...radead.org>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: Christoph Hellwig <hch@...radead.org>,
	Zhang Yi <yi.zhang@...weicloud.com>, linux-xfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	brauner@...nel.org, david@...morbit.com, chandanbabu@...nel.org,
	jack@...e.cz, willy@...radead.org, yi.zhang@...wei.com,
	chengzhihao1@...wei.com, yukuai3@...wei.com
Subject: Re: [RFC PATCH v4 8/8] xfs: improve truncate on a realtime inode
 with huge extsize

On Fri, May 31, 2024 at 07:12:10AM -0700, Darrick J. Wong wrote:
> There are <cough> some users that want 1G extents.
> 
> For the rest of us who don't live in the stratosphere, it's convenient
> for fsdax to have rt extents that match the PMD size, which could be
> large on arm64 (e.g. 512M, or two smr sectors).

That's fine.  Maybe to rephrase my question.  With this series we
have 3 different truncate path:

 1) unmap all blocks (!rt || rtextsizse == 1)
 2) zero leftover blocks in an rtextent (small rtextsize, but > 1)
 3) converted leftover block in an rtextent to unwritten (large
   rtextsize)

What is the right threshold to switch between 2 and 3?  And do we
really need 2) at all?