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From: Matthew Wilcox <>
To: Jan Kara <>
Cc:,,, Ted Tso <>,
	Christoph Hellwig <>,
	Amir Goldstein <>,
	Dave Chinner <>
Subject: Re: [PATCH 0/7 RFC v3] fs: Hole punch vs page cache filling races
Date: Mon, 19 Apr 2021 16:20:08 +0100
Message-ID: <> (raw)
In-Reply-To: <>

On Tue, Apr 13, 2021 at 01:28:44PM +0200, Jan Kara wrote:
> Also when writing the documentation I came across one question: Do we mandate
> i_mapping_sem for truncate + hole punch for all filesystems or just for
> filesystems that support hole punching (or other complex fallocate operations)?
> I wrote the documentation so that we require every filesystem to use
> i_mapping_sem. This makes locking rules simpler, we can also add asserts when
> all filesystems are converted. The downside is that simple filesystems now pay
> the overhead of the locking unnecessary for them. The overhead is small
> (uncontended rwsem acquisition for truncate) so I don't think we care and the
> simplicity is worth it but I wanted to spell this out.

What do we actually get in return for supporting these complex fallocate
operations?  Someone added them for a reason, but does that reason
actually benefit me?  Other than running xfstests, how many times has
holepunch been called on your laptop in the last week?  I don't want to
incur even one extra instruction per I/O operation to support something
that happens twice a week; that's a bad tradeoff.

Can we implement holepunch as a NOP?  Or return -ENOTTY?  Those both
seem like better solutions than adding an extra rwsem to every inode.
Failing that, is there a bigger hammer we can use on the holepunch side
(eg preventing all concurrent accesses while the holepunch is happening)
to reduce the overhead on the read side?

  parent reply index

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-13 11:28 Jan Kara
2021-04-13 11:28 ` [PATCH 1/7] mm: Fix comments mentioning i_mutex Jan Kara
2021-04-13 12:38   ` Christoph Hellwig
2021-04-13 11:28 ` [PATCH 2/7] mm: Protect operations adding pages to page cache with i_mapping_lock Jan Kara
2021-04-13 12:57   ` Christoph Hellwig
2021-04-13 13:56     ` Jan Kara
2021-04-14  0:01   ` Dave Chinner
2021-04-14 12:23     ` Jan Kara
2021-04-14 21:57       ` Dave Chinner
2021-04-15 13:11         ` Jan Kara
2021-04-14 22:25     ` Matthew Wilcox
2021-04-15  2:05       ` Dave Chinner
2021-04-13 11:28 ` [PATCH 3/7] ext4: Convert to use inode->i_mapping_sem Jan Kara
2021-04-13 11:28 ` [PATCH 4/7] ext2: Convert to using i_mapping_sem Jan Kara
2021-04-13 11:28 ` [PATCH 5/7] xfs: Convert to use i_mapping_sem Jan Kara
2021-04-13 13:05   ` Christoph Hellwig
2021-04-13 13:42     ` Jan Kara
2021-04-13 11:28 ` [PATCH 6/7] zonefs: Convert to using i_mapping_sem Jan Kara
2021-04-13 11:28 ` [PATCH 7/7] fs: Remove i_mapping_sem protection from .page_mkwrite handlers Jan Kara
2021-04-13 13:09 ` [PATCH 0/7 RFC v3] fs: Hole punch vs page cache filling races Christoph Hellwig
2021-04-13 14:17   ` Jan Kara
2021-04-19 15:20 ` Matthew Wilcox [this message]
2021-04-19 16:25   ` Jan Kara
2021-04-20 22:12   ` Dave Chinner

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