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Date: Wed, 27 Mar 2024 18:45:17 +0000
From: Matthew Wilcox <willy@infradead.org>
To: David Howells <dhowells@redhat.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>,
	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, devel@lists.orangefs.org,
	linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH v2] mm, netfs: Provide a means of invalidation
 without using launder_folio
Message-ID: <ZgRpPd1Ado-0_iYx@casper.infradead.org>
References: <2318298.1711551844@warthog.procyon.org.uk>
 <2506007.1711562145@warthog.procyon.org.uk>
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On Wed, Mar 27, 2024 at 05:55:45PM +0000, David Howells wrote:
> +int filemap_invalidate_inode(struct inode *inode, bool flush)
> +{
> +	struct address_space *mapping = inode->i_mapping;
> +
> +	if (!mapping || !mapping->nrpages)
> +		goto out;
> +
> +	/* Prevent new folios from being added to the inode. */
> +	filemap_invalidate_lock(mapping);

I'm kind of surprised that the callers wouldn't want to hold that lock
over a call to this function.  I guess you're working on the callers,
so you'd know better than I would, but I would have used lockdep to
assert that invalidate_lock was held.

> +	if (!mapping->nrpages)
> +		goto unlock;
> +
> +	/* Assume there are probably PTEs only if there are mmaps. */
> +	if (unlikely(!RB_EMPTY_ROOT(&mapping->i_mmap.rb_root)))
> +		unmap_mapping_pages(mapping, 0, ULONG_MAX, false);

Is this optimisation worth it?  We're already doing some expensive
operations here, does saving cycling the i_mmap_lock really help
anything?  You'll note that unmap_mapping_pages() already does this
check inside the lock.