Re: [PATCH] mm for fs: add truncate_pagecache_range

[Joel Becker]

On Sun, Mar 25, 2012 at 01:26:10PM -0700, Hugh Dickins wrote:
> On Fri, 23 Mar 2012, Andrew Morton wrote:
> > On Fri, 23 Mar 2012 14:14:54 -0700 (PDT)
> > Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> > > On Fri, 23 Mar 2012, Andrew Morton wrote:
> > > > 
> > > > --- a/mm/truncate.c~mm-for-fs-add-truncate_pagecache_range-fix
> > > > +++ a/mm/truncate.c
> > > > @@ -639,6 +639,9 @@ int vmtruncate_range(struct inode *inode
> > > >   * with on-disk format, and the filesystem would not have to deal with
> > > >   * situations such as writepage being called for a page that has already
> > > >   * had its underlying blocks deallocated.
> > > > + *
> > > > + * Must be called with inode->i_mapping->i_mutex held.
> > > 
> > > You catch me offguard: I forget whether that's an absolute requirement or
> > > just commonly the case.  What do the other interfaces in truncate.c say ?-)
> > 
> > i_mutex is generally required, to stabilise i_size.
> 
> Sorry for being quarrelsome, but I do want to Nak your followup "fix".
> 
> Building a test kernel quickly told me that inode->i_mapping->i_mutex
> doesn't exist, of course it's inode->i_mutex.
> 
> Then running the test kernel quickly told me that neither ext4 nor xfs
> (I didn't try ocfs2) holds inode->i_mutex where holepunching calls
> truncate_inode_pages_range().

Just for completeness:

ocfs2 holds i_mutex around the entire ocfs2_change_file_space() call,
which can do hole punching and unwritten extent allocation (it is a
clone of xfs_change_file_space()).  xfs itself seems hold its own idea
of a shared lock while doing the work and an exclusive lock around the
transaction join.  I'm not clear enough about xfs to say how this
compares or even if I read it right.

But ocfs2 uses an allocation sem to protect allocation changes,
including i_size, so perhaps i_mutex isn't strictly necessary.  I don't
think we contend enough here to try hard to remove it :-)

Joel

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