Re: [RFC PATCH v2] mm, netfs: Provide a means of invalidation without using launder_folio

[Matthew Wilcox]

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.