Re: [PATCH] filemap: Fix error propagation in do_read_cache_page()

[Alexander Larsson <alexl@redhat.com>]

On Wed, Sep 21, 2022 at 7:32 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> On Wed, Sep 21, 2022 at 11:10:10AM +0200, Alexander Larsson wrote:
> > When do_read_cache_folio() returns an error pointer the code
> > was dereferencing it rather than forwarding the error via
> > ERR_CAST().
> >
> > Found during code review.
> >
> > Fixes: 539a3322f208 ("filemap: Add read_cache_folio and read_mapping_folio")
> > Signed-off-by: Alexander Larsson <alexl@redhat.com>
> > ---
> >  mm/filemap.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 15800334147b..6bc55506f7a8 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -3560,7 +3560,7 @@ static struct page *do_read_cache_page(struct address_space *mapping,
> >
> >       folio = do_read_cache_folio(mapping, index, filler, file, gfp);
> >       if (IS_ERR(folio))
> > -             return &folio->page;
> > +             return ERR_CAST(folio);
>
> Where do you see a dereference?  I agree that your variant is cleaner,
> but &folio->page does *NOT* dereference anything - it's an equivalent of
>
>         (struct page *)((unsigned long)folio + offsetof(struct folio, page))
>
> and the reason it happens to work is that page is the first member in
> struct folio, so the offsetof ends up being 0 and we are left with a cast
> from struct folio * to struct page *, i.e. the same thing ERR_CAST()
> variant end up with (it casts to void *, which is converted to struct
> page * since return acts as assignment wrt type conversions).
>
> It *is* brittle and misguiding, and your patch is a much more clear
> way to spell that thing, no arguments about it; just that your patch
> is not changing behaviour.

Yeah, it doesn't actually dereference, but what I was thinking is that
the caller could dereference it, if the addition made it not an error.
However, I didn't look at the actual offset of page in folio, so
you're right, this is actually fine.

Still, better change this to avoid confusing people.

-- 
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 Alexander Larsson                                Red Hat, Inc
       alexl@redhat.com         alexander.larsson@gmail.com