Re: [PATCH] fs: reduce pointers while using file_ra_state_init()

["NeilBrown" <neilb@suse.de>]

On Tue, 27 Jul 2021, Matthew Wilcox wrote:
> On Tue, Jul 27, 2021 at 08:06:21AM +1000, NeilBrown wrote:
> > You seem to be assuming that inode->i_mapping->host is always 'inode'.
> > That is not the case.
> 
> Weeeelllll ... technically, outside of the filesystems that are
> changed here, the only assumption in common code that is made is that
> inode_to_bdi(inode->i_mapping->host->i_mapping->host) ==
> inode_to_bdi(inode)

Individual filesystems doing their own thing is fine.  Passing just an
inode to inode_to_bdi is fine.

But the patch changes do_dentry_open()

> 
> Looking at inode_to_bdi, that just means that they have the same i_sb.
> Which is ... not true for character raw devices?
>         if (++raw_devices[minor].inuse == 1)
>                 file_inode(filp)->i_mapping =
>                         bdev->bd_inode->i_mapping;
> but then, who's using readahead on a character raw device?  They
> force O_DIRECT.  But maybe this should pass inode->i_mapping->host
> instead of inode.

Also not true in coda.

coda (for those who don't know) is a network filesystem which fetches
whole files (and often multiple files) at a time (like the Andrew
filesystem).  The files are stored in a local filesystem which acts as a
cache.

So an inode in a 'coda' filesystem access page-cache pages from a file
in e.g. an 'ext4' filesystem.  This is done via the ->i_mapping link.
For (nearly?) all other filesystems, ->i_mapping is a link to ->i_data
in the same inode.

> 
> > In particular, fs/coda/file.c contains
> > 
> > 	if (coda_inode->i_mapping == &coda_inode->i_data)
> > 		coda_inode->i_mapping = host_inode->i_mapping;
> > 
> > So a "coda_inode" shares the mapping with a "host_inode".
> > 
> > This is why an inode has both i_data and i_mapping.
> > 
> > So I'm not really sure this patch is safe.  It might break codafs.
> > 
> > But it is more likely that codafs isn't used, doesn't work, should be
> > removed, and i_data should be renamed to i_mapping.
> 
> I think there's also something unusual going on with either ocfs2
> or gfs2.  But yes, I don't understand the rules for when I need to
> go from inode->i_mapping->host.
> 

Simple.  Whenever you want to work with the page-cache pages, you cannot
assume anything in the original inode is relevant except i_mapping (and
maybe i_size I guess).

NeilBrown