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

[Matthew Wilcox <willy@infradead.org>]

On Wed, Sep 21, 2022 at 06:16:18PM +0100, Al Viro 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.

I don't see it that way.  &folio->page is the idiomatic way to do this.
What it really is, is an indicator that code needs to be converted from
calling do_read_cache_page() and friends to calling the folio equivalents.
Also, I should have moved this code to folio-compat.c where it would be
with all the other code that uses this idiom.