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

[Nhat Pham]

On Thu, Jun 25, 2026 at 5:53 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> 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.

Ah I see! Admittedly, I'm not aware with this POSIX detail. Thanks for
the clarifications and pointers, Matthew!

In that case, I have no further comments on this patch, and defer to
overlayfs and fs folks for FS-specific correctness.

Reviewed-by: Nhat Pham <nphamcs@xxxxxxxxx>