Re: [PATCH v2] erofs: use dedicated meta inodes for file-backed mounts

[Christoph Hellwig]

On Thu, Aug 20, 2026 at 09:40:46PM +0800, Gao Xiang wrote:
> Hi Christoph,
> 
> On Tue, Aug 18, 2026 at 10:55:47PM -0700, Christoph Hellwig wrote:
> > On Tue, Aug 11, 2026 at 05:07:17PM +0800, Gao Xiang wrote:
> > > access on file-backed mounts (although I still don't think it is
> > > necessary due to the EROFS immutable model). As the result, metadata
> > > cache won't use the page cache of backing inodes anymore.
> > > 
> > > The "managed cache" was originally used to cache physical compressed
> > > data according to the current cache strategy and I/O patterns; since
> > > file-backed mounts also need to access physical data for metadata
> > > access, it's natural to reuse the managed cache for this too, providing
> > > a unique inode for all physical data access.
> > 
> > This look sane.  Any reason to not use the meta inode unconditionally
> > to simplify the code?
> 
> I guess you meant avoid using bd_inode page cache?


Yes.

> I'm not sure if it simplifies a lot, also like fadvise(WILLNEED) to blkdev
> can perform to bd_inode too so I think it can still have some benefits.

This is actually an artefact of why using the bdev page cache is a bad
idea, because it opens your metadata cache up to external modification
and side channel attacks.

> I think bd_inode page cache can be used for filesystems so maybe leave it
> as-is for now, or maybe a follow-up work if it's really needed.

That's how it has historically been done.  We generally try to avoid
it for newer designs.