Re: UML/hostfs - mount failure at tip of tree

[Hongbo Li]

On 2024/7/24 9:14, Linus Torvalds wrote:
> On Tue, 23 Jul 2024 at 15:33, Maciej Żenczykowski <maze@xxxxxxxxxx> wrote:
> > Reverting the following 3 patches:
> > - 104eef133fd9 hostfs: Add const qualifier to host_root in hostfs_fill_super()
> > - cd140ce9f611 hostfs: convert hostfs to use the new mount API
> > - e3ec0fe944d2 hostfs: Convert hostfs_read_folio() to use a folio
> >
> > appears to be necessary to get the Android net test framework to boot
> > with tip of tree,
> > *without* the reverts we get:
> >    mount: /host: special device hostfs does not exist.
> > (if I don't revert the folio change then it mounts, but appears to not
> > actually work)
>
> Interesting. That folio change was clearly supposed to be a no-op, but
> isn't. Which makes a revert the right thing to do regardless.
>
> That code was odd before too, but clearly that commit is completely broken.
>
> I think this part is buggy:
>
>                  buffer = folio_zero_tail(folio, bytes_read, buffer);
>
> because while the documentation for folio_zero_tail() does imply that
> usage, the third argument is supposed really looks like it should be
> "buffer + bytes_read".
>
> So  instead of reverting that commit, does it help to just do that instead:
>
> -               buffer = folio_zero_tail(folio, bytes_read, buffer);
> +               buffer = folio_zero_tail(folio, bytes_read, buffer +
> bytes_read);
>
> Willy, that function is really bad. It's not helpful when it
> apparently confused even you, and the calling convention really is
> broken. I think that folio_zero_tail() needs to be rewritten to have
> sane calling conventions (like matching the docs!) or just die.
>
> The mount API change is somethign else. Again, it wasn't supposed to
> break anything, but clearly does, and so reverting it sounds sane
> unless somebody sees what the problem is.
I apologize for causing this issue. I am currently tracking it down.  If 
reverting this can solve the problem, you can revert it first.

Thanks,
Hongbo

> I'm not even guessing at what might have been wrong in that mount API
> conversion.
>
>             Linus