Re: [PATCH 1/3] mm: Add folio_zero_tail() and use it in ext4

Andrew Morton
2023-11-09T17:27:21Z

On Thu, 9 Nov 2023 01:12:15 +0100 Andreas Grünbacher <andreas.gruenbacher@gmail.com> wrote:

> Andrew,
>
> Andrew Morton <akpm@linux-foundation.org> schrieb am Do., 9. Nov. 2023, 00:06:
> > > +
> > > + if (folio_test_highmem(folio)) {
> > > + size_t max = PAGE_SIZE - offset_in_page(offset);
> > > +
> > > + while (len > max) {
> >
> > Shouldn't this be `while (len)'? AFAICT this code can fail to clear
> > the final page.
>
> not sure what you're seeing there, but this looks fine to me.

I was right! This code does fail to handle the final page.

: 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);

This code down here handles it, doh.

: to = folio_zero_tail(folio, offset, to);
: kunmap_local(to);
: }

Implementation seems less straightforward than it might be? Oh well.

Has it been runtime tested?

Anyway, let's please change the function argument ordering and remember
to cc linux-mm on v2?