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From: Christoph Hellwig <hch@lst.de>
To: David Howells <dhowells@redhat.com>
Cc: linux-fsdevel@vger.kernel.org, viro@zeniv.linux.org.uk,
	hch@lst.de, tytso@mit.edu, adilger.kernel@dilger.ca,
	darrick.wong@oracle.com, clm@fb.com, josef@toxicpanda.com,
	dsterba@suse.com, linux-ext4@vger.kernel.org,
	linux-xfs@vger.kernel.org, linux-btrfs@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: Problems with determining data presence by examining extents?
Date: Wed, 15 Jan 2020 09:38:54 +0100
Message-ID: <20200115083854.GB23039@lst.de> (raw)
In-Reply-To: <4467.1579020509@warthog.procyon.org.uk>

On Tue, Jan 14, 2020 at 04:48:29PM +0000, David Howells wrote:
> Again with regard to my rewrite of fscache and cachefiles:
> 
> 	https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=fscache-iter
> 
> I've got rid of my use of bmap()!  Hooray!
> 
> However, I'm informed that I can't trust the extent map of a backing file to
> tell me accurately whether content exists in a file because:
> 
>  (a) Not-quite-contiguous extents may be joined by insertion of blocks of
>      zeros by the filesystem optimising itself.  This would give me a false
>      positive when trying to detect the presence of data.
> 
>  (b) Blocks of zeros that I write into the file may get punched out by
>      filesystem optimisation since a read back would be expected to read zeros
>      there anyway, provided it's below the EOF.  This would give me a false
>      negative.

The whole idea of an out of band interface is going to be racy and suffer
from implementation loss.  I think what you want is something similar to
the NFSv4.2 READ_PLUS operation - give me that if there is any and
otherwise tell me that there is a hole.  I think this could be a new
RWF_NOHOLE or similar flag, just how to return the hole size would be
a little awkward.  Maybe return a specific negative error code (ENODATA?)
and advance the iov anyway.

  parent 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
2020-01-17 16:43           ` David Howells
2020-01-15 14:20   ` David Howells
2020-01-15  8:38 ` Christoph Hellwig [this message]
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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