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From: Miklos Szeredi <miklos@szeredi.hu>
Date: Tue, 19 Mar 2024 17:40:58 +0100
Message-ID: <CAJfpegsCBEm11OHS8bfQdgossOgofPcYhLTFtw7_+T66iBvznw@mail.gmail.com>
Subject: Re: [RFC PATCH] mm: Replace ->launder_folio() with flush and wait
To: David Howells <dhowells@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>, Trond Myklebust <trond.myklebust@hammerspace.com>, 
	Christoph Hellwig <hch@lst.de>, Andrew Morton <akpm@linux-foundation.org>, 
	Alexander Viro <viro@zeniv.linux.org.uk>, Christian Brauner <brauner@kernel.org>, 
	Jeff Layton <jlayton@kernel.org>, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, 
	netfs@lists.linux.dev, v9fs@lists.linux.dev, linux-afs@lists.infradead.org, 
	ceph-devel@vger.kernel.org, linux-cifs@vger.kernel.org, 
	linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"

On Tue, 19 Mar 2024 at 17:13, Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> On Tue, 19 Mar 2024 at 15:15, David Howells <dhowells@redhat.com> wrote:
>
> > What particular usage case of invalidate_inode_pages2() are you thinking of?
>
> FUSE_NOTIFY_INVAL_INODE will trigger invalidate_inode_pages2_range()
> to clean up the cache.
>
> The server is free to discard writes resulting from this invalidation
> and delay reads in the region until the invalidation finishes.  This
> would no longer work with your change, since the mapping could
> silently be reinstated between the writeback and the removal from the
> cache due to the page being unlocked/relocked.

This would also matter if a distributed filesystem wanted to implement
coherence even if there are mmaps.   I.e. a client could get exclusive
access to a region by issuing FUSE_NOTIFY_INVAL_INODE on all other
clients and blocking reads.  With your change this would fail.

Again, this is purely theoretical, and without a way to differentiate
between the read-only and write cases it has limited usefulness.
Adding leases to fuse (which I plan to do) would make this much more
useful.

Thanks,
Miklos