[PATCH 2/3] mm: Add folio_fill_tail() and use it in iomap

[Andreas Grünbacher]

Am Fr., 10. Nov. 2023 um 18:09 Uhr schrieb Matthew Wilcox <willy at infradead.org>:
> On Thu, Nov 09, 2023 at 10:50:45PM +0100, Andreas Gruenbacher wrote:
> > On Tue, Nov 7, 2023 at 10:27 PM Matthew Wilcox (Oracle)
> > <willy at infradead.org> wrote:
> > > +static inline void folio_fill_tail(struct folio *folio, size_t offset,
> > > +               const char *from, size_t len)
> > > +{
> > > +       char *to = kmap_local_folio(folio, offset);
> > > +
> > > +       VM_BUG_ON(offset + len > folio_size(folio));
> > > +
> > > +       if (folio_test_highmem(folio)) {
> > > +               size_t max = PAGE_SIZE - offset_in_page(offset);
> > > +
> > > +               while (len > max) {
> > > +                       memcpy(to, from, max);
> > > +                       kunmap_local(to);
> > > +                       len -= max;
> > > +                       from += max;
> > > +                       offset += max;
> > > +                       max = PAGE_SIZE;
> > > +                       to = kmap_local_folio(folio, offset);
> > > +               }
> > > +       }
> > > +
> > > +       memcpy(to, from, len);
> > > +       to = folio_zero_tail(folio, offset, to);
> >
> > This needs to be:
> >
> > to = folio_zero_tail(folio, offset  + len, to + len);
>
> Oh, wow, that was stupid of me.  I only ran an xfstests against ext4,
> which doesn't exercise this code, not gfs2 or erofs.  Thanks for
> fixing this up.
>
> I was wondering about adding the assertion:
>
>         VM_BUG_ON((kaddr - offset) % PAGE_SIZE);
>
> to catch the possible mistake of calling kmap_local_folio(folio, 0)
> instead of kmap_local_folio(folio, offset).  But maybe that's
> sufficiently unlikely a mistake to bother adding a runtime check for.

folio_zero_tail() is a bit of an obscure function, so I'm not sure if
there will be additional callers. The parameters are described as:

 * @offset: The byte offset in the folio to start zeroing at.
 * @kaddr: The address the folio is currently mapped to.

What about changing the @kaddr description to 'the (mapped) address
within the folio to start zeroing at' or similar?

Andreas