linux-kernel - Re: Problems with determining data presence by examining extents?

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Message-ID: <20200116101344.GA16435@lst.de>
Date:   Thu, 16 Jan 2020 11:13:44 +0100
From:   Christoph Hellwig <hch@....de>
To:     David Howells <dhowells@...hat.com>
Cc:     Christoph Hellwig <hch@....de>, Qu Wenruo <quwenruo.btrfs@....com>,
        Andreas Dilger <adilger@...ger.ca>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-btrfs <linux-btrfs@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Problems with determining data presence by examining extents?

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.