Re: [RFC PATCH] mm: Replace ->launder_folio() with flush and wait

From: Matthew Wilcox <willy@infradead.org>
To: David Howells <dhowells@redhat.com>
Cc: 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] mm: Replace ->launder_folio() with flush and wait
Date: Thu, 7 Mar 2024 04:45:19 +0000	[thread overview]
Message-ID: <ZelGX3vVlGfEZm8H@casper.infradead.org> (raw)
In-Reply-To: <1668172.1709764777@warthog.procyon.org.uk>

On Wed, Mar 06, 2024 at 10:39:37PM +0000, David Howells wrote:
> Here's a patch to have a go at getting rid of ->launder_folio().  Since it's
> failable and cannot guarantee that pages in the range are removed, I've tried
> to replace laundering with just flush-and-wait, dropping the folio lock around
> the I/O.

My sense is that ->launder_folio doesn't actually need to be replaced.

commit e3db7691e9f3dff3289f64e3d98583e28afe03db
Author: Trond Myklebust <Trond.Myklebust@netapp.com>
Date:   Wed Jan 10 23:15:39 2007 -0800

    [PATCH] NFS: Fix race in nfs_release_page()

        NFS: Fix race in nfs_release_page()

        invalidate_inode_pages2() may find the dirty bit has been set on a page
        owing to the fact that the page may still be mapped after it was locked.
        Only after the call to unmap_mapping_range() are we sure that the page
        can no longer be dirtied.
        In order to fix this, NFS has hooked the releasepage() method and tries
        to write the page out between the call to unmap_mapping_range() and the
        call to remove_mapping(). This, however leads to deadlocks in the page
        reclaim code, where the page may be locked without holding a reference
        to the inode or dentry.

        Fix is to add a new address_space_operation, launder_page(), which will
        attempt to write out a dirty page without releasing the page lock.

    Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

I don't understand why this couldn't've been solved by page_mkwrite.
NFS did later add nfs_vm_page_mkwrite in July 2007, and maybe it's just
not needed any more?  I haven't looked into it enough to make sure,
but my belief is that we should be able to get rid of it.