linux-ext4 - Re: [PATCH 08/15] fsverity: kick off hash readahead at data I/O submission time

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Message-ID: <20260130055157.GB622@lst.de>
Date: Fri, 30 Jan 2026 06:51:57 +0100
From: Christoph Hellwig <hch@....de>
To: Eric Biggers <ebiggers@...nel.org>
Cc: Christoph Hellwig <hch@....de>, Al Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	David Sterba <dsterba@...e.com>, Theodore Ts'o <tytso@....edu>,
	Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>,
	Andrey Albershteyn <aalbersh@...hat.com>,
	Matthew Wilcox <willy@...radead.org>, linux-fsdevel@...r.kernel.org,
	linux-btrfs@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-f2fs-devel@...ts.sourceforge.net, fsverity@...ts.linux.dev
Subject: Re: [PATCH 08/15] fsverity: kick off hash readahead at data I/O
 submission time

On Wed, Jan 28, 2026 at 02:56:02PM -0800, Eric Biggers wrote:
> On Wed, Jan 28, 2026 at 04:26:20PM +0100, Christoph Hellwig wrote:
> > Currently all reads of the fsverity hashes is kicked off from the data
> > I/O completion handler, leading to needlessly dependent I/O.  This is
> > worked around a bit by performing readahead on the level 0 nodes, but
> > still fairly ineffective.
> > 
> > Switch to a model where the ->read_folio and ->readahead methods instead
> > kick off explicit readahead of the fsverity hashed so they are usually
> > available at I/O completion time.
> > 
> > For 64k sequential reads on my test VM this improves read performance
> > from 2.4GB/s - 2.6GB/s to 3.5GB/s - 3.9GB/s.  The improvements for
> > random reads are likely to be even bigger.
> > 
> > Signed-off-by: Christoph Hellwig <hch@....de>
> > Acked-by: David Sterba <dsterba@...e.com> [btrfs]
> 
> Unfortunately, this patch causes recursive down_read() of
> address_space::invalidate_lock.  How was this meant to work?

It worked by the chances that multiple down_read generally work.
Except when they don't and we have a write queued up in between,
but nothing in xfstests hits that.

I'll look into reworking it to avoid that.