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Date: Thu, 29 Aug 2024 06:00:56 +0200
From: Christoph Hellwig <hch@lst.de>
To: "Darrick J. Wong" <djwong@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>, linux-fsdevel@vger.kernel.org,
	Dave Chinner <david@fromorbit.com>,
	"Darrick J. Wong" <darrick.wong@oracle.com>,
	Christoph Hellwig <hch@lst.de>,
	Chuck Lever <chuck.lever@oracle.com>, Jan Kara <jack@suse.cz>
Subject: Re: VFS caching of file extents
Message-ID: <20240829040056.GA4142@lst.de>
References: <Zs97qHI-wA1a53Mm@casper.infradead.org> <20240829015750.GB6216@frogsfrogsfrogs>
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User-Agent: Mutt/1.5.17 (2007-11-01)

> Wouldn't readplus (and maybe a sparse copy program) rather have
> something that is "SEEK_DATA, fill the buffer with data from that file
> position, and tell me what pos the data came from"?

Or rather a read operation that returns a length but no data if there
is a hole.  Either way a potentially incoherent VFS cache is the wrong
way to implement it.

> I also suspect that devising a "simple" mapping tree for simple
> filesystems will quickly devolve into a mess of figuring out their adhoc
> locking and making that work.

Heh.  If simple really is the file systems just using the buffer_head
helpers without any magic it might actually not be that bad, but once
it gets a little more complicated I tend to agree.

But the first thing would be to establish if we actually need it at all,
or if the buffer_head caching of their metadata is actually enough for
the file systems.