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From: Christoph Hellwig <hch@lst.de>
To: David Howells <dhowells@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>,
	Qu Wenruo <quwenruo.btrfs@gmx.com>,
	Andreas Dilger <adilger@dilger.ca>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	"Theodore Y. Ts'o" <tytso@mit.edu>,
	"Darrick J. Wong" <darrick.wong@oracle.com>,
	Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
	David Sterba <dsterba@suse.com>,
	linux-ext4 <linux-ext4@vger.kernel.org>,
	linux-xfs <linux-xfs@vger.kernel.org>,
	linux-btrfs <linux-btrfs@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: Problems with determining data presence by examining extents?
Date: Thu, 16 Jan 2020 11:13:44 +0100
Message-ID: <20200116101344.GA16435@lst.de> (raw)
In-Reply-To: <28755.1579100378@warthog.procyon.org.uk>

On Wed, Jan 15, 2020 at 02:59:38PM +0000, David Howells wrote:
> Another thread could be writing to the file at the same time, but not in the
> same block.  That's managed by netfs, most likely based on the pages and page
> flags attached to the netfs inode being cached in this particular file[*].
> 
> What I was more thinking of is that SEEK_HOLE might run past the block of
> interest and into a block that's currently being written and see a partially
> written block.

But that's not a problem given that you know where to search.

> 
> [*] For AFS, this is only true of regular files; dirs and symlinks are cached
>     as monoliths and are there entirely or not at all.
> 
> > > However, SEEK_HOLE doesn't help with the issue of the filesystem 'altering'
> > > the content of the file by adding or removing blocks of zeros.
> > 
> > As does any other method.  If you need that fine grained control you
> > need to track the information yourself.
> 
> So, basically, I can't.  Okay.  I was hoping it might be possible to add an
> ioctl or something to tell filesystems not to do that with particular files.

File systems usually pad zeroes where they have to, typically for
sub-blocksize writes.   Disabling this would break data integrity.

  reply index

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-14 16:48 David Howells
2020-01-14 22:49 ` Theodore Y. Ts'o
2020-01-15  3:54 ` Qu Wenruo
2020-01-15 12:46   ` Andreas Dilger
2020-01-15 13:10     ` Qu Wenruo
2020-01-15 13:31       ` Christoph Hellwig
2020-01-15 19:48         ` Andreas Dilger
2020-01-16 10:16           ` Christoph Hellwig
2020-01-15 20:55         ` David Howells
2020-01-15 22:11           ` Andreas Dilger
2020-01-15 23:09           ` David Howells
2020-01-26 18:19             ` Zygo Blaxell
2020-01-15 14:35       ` David Howells
2020-01-15 14:48         ` Christoph Hellwig
2020-01-15 14:59         ` David Howells
2020-01-16 10:13           ` Christoph Hellwig [this message]
2020-01-17 16:43           ` David Howells
2020-01-15 14:20   ` David Howells
2020-01-15  8:38 ` Christoph Hellwig
2020-01-15 13:50 ` David Howells
2020-01-15 14:05 ` David Howells
2020-01-15 14:24   ` Qu Wenruo
2020-01-15 14:50   ` David Howells
2020-01-15 14:15 ` David Howells

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