Re: [PATCH] netfs: Fix lockdep warning from taking sb_writers whilst holding mmap_lock

[Matthew Wilcox]

On Tue, Dec 07, 2021 at 11:19:35AM +0000, David Howells wrote:
> Taking sb_writers whilst holding mmap_lock isn't allowed and will result in
> a lockdep warning like that below.  The problem comes from cachefiles
> needing to take the sb_writers lock in order to do a write to the cache,
> but being asked to do this by netfslib called from readpage, readahead or
> write_begin[1].

Isn't it taking sb_writers _on a different filesystem_?  So there's not
a real deadlock here, just a need to tell lockdep that this is a
different subclass of lock?