linux-kernel - Re: [PATCH 4/4] xfs: fallback to buffered I/O for direct I/O when stable writes are required

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20251029163555.GB26985@lst.de>
Date: Wed, 29 Oct 2025 17:35:55 +0100
From: Christoph Hellwig <hch@....de>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: Christoph Hellwig <hch@....de>, Carlos Maiolino <cem@...nel.org>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-raid@...r.kernel.org,
	linux-block@...r.kernel.org, Qu Wenruo <wqu@...e.com>,
	linux-btrfs@...r.kernel.org
Subject: Re: [PATCH 4/4] xfs: fallback to buffered I/O for direct I/O when
 stable writes are required

[Adding Qu and the btrfs list]

On Wed, Oct 29, 2025 at 08:53:06AM -0700, Darrick J. Wong wrote:
> > +	if (mapping_stable_writes(iocb->ki_filp->f_mapping)) {
> > +		xfs_info_once(mp,
> > +			"falling back from direct to buffered I/O for write");
> > +		return -ENOTBLK;
> > +	}
> 
> /me wonders if the other filesystems will have to implement this same
> fallback and hence this should be a common helper ala
> dio_warn_stale_pagecache?  But we'll get there when we get there.

As far as I'm concerned they should.  Btrfs in fact has recently done
that, as they are even more exposed due to the integrated checksumming.

So yes, a common helper might make sense.  Especially if we want common
configuration for opt-outs eventually.

> >  	file->f_mode |= FMODE_NOWAIT | FMODE_CAN_ODIRECT;
> > -	file->f_mode |= FMODE_DIO_PARALLEL_WRITE;
> > -	if (xfs_get_atomic_write_min(XFS_I(inode)) > 0)
> > -		file->f_mode |= FMODE_CAN_ATOMIC_WRITE;
> > +	if (!mapping_stable_writes(file->f_mapping)) {
> > +		file->f_mode |= FMODE_DIO_PARALLEL_WRITE;
> 
> Hrm.  So parallel directio writes are disabled for writes to files on
> stable_pages devices because we have to fall back to buffered writes.
> Those serialize on i_rwsem so that's why we don't set
> FMODE_DIO_PARALLEL_WRITE, correct?

Yes.

> There's not some more subtle reason
> for not supporting it, right?

Not that I know of anyway.