linux-kernel - Re: Efficient mapping of sparse file holes to zero-pages

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Message-ID: <Z7ybjloC5fobHbTE@infradead.org>
Date: Mon, 24 Feb 2025 08:17:18 -0800
From: Christoph Hellwig <hch@...radead.org>
To: Matthew Wilcox <willy@...radead.org>
Cc: David Frank <david@...idfrank.ch>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: Efficient mapping of sparse file holes to zero-pages

On Sun, Feb 23, 2025 at 01:47:52AM +0000, Matthew Wilcox wrote:
>  - Choose a data structure in the VFS to store this range information
>    (a tree of some kind)
>  - Design a protocol such that the VFS can query this information about
>    a range of a particular file, and the filesystem can invalidate the
>    VFS's knowledge

That information is always going to be incoherent in some way.  Reads
are already done without i_rwsem for most file systems and there is
further work on reducing locking.  So anything needs to come from inside
the file system itself.  That probably means you can't reduce the folio
allocation overhead, but at least you don't have to persistently use the
memory.