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

[Matthew Wilcox]

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.