Re: [PATCH 2/3] ovl: support cachestat() syscall on overlayfs files

[Matthew Wilcox <willy@infradead.org>]

On Wed, Jun 24, 2026 at 12:06:54PM -0700, Nhat Pham wrote:
> I'm more concerned with undocumented/unexpected behavior (error type
> in this case). -EIO was an example that I saw in ovl_real_file()
> itself, but I'm not familiar enough with overlayfs to know if that's
> the extent of it.
> 
> But I'm OK with just updating the documentation with a simple note
> that other error maybe propagated from the underlying fs, if no one
> else thinks it's a problem :)

That's ALWAYS true.  POSIX even says so explicitly in section 2.3:

  Implementations may support additional errors not included in this list,
  may generate errors included in this list under circumstances other
  than those described here, or may contain extensions or limitations
  that prevent some errors from occurring.

We don't generally bother to document that pretty much every syscall may
return -ENOMEM if it can't allocate memory.  That's just ... expected.
open(2) documents the possibility, but read(2) doesn't.  I think it's
the same for EIO.  Any operation which accesses storage can return -EIO.