Re: [PATCH 3/6] gfs2: Convert gfs2_write_jdata_page() to gfs2_write_jdata_folio()

[Matthew Wilcox <willy@infradead.org>]

On Sat, Jun 03, 2023 at 11:34:14AM +0200, Andreas Gruenbacher wrote:
> >   * This is the same as calling block_write_full_page, but it also
> >   * writes pages outside of i_size
> >   */
> > -static int gfs2_write_jdata_page(struct page *page,
> > +static int gfs2_write_jdata_folio(struct folio *folio,
> >                                  struct writeback_control *wbc)
> >  {
> > -       struct inode * const inode = page->mapping->host;
> > +       struct inode * const inode = folio->mapping->host;
> >         loff_t i_size = i_size_read(inode);
> > -       const pgoff_t end_index = i_size >> PAGE_SHIFT;
> > -       unsigned offset;
> >
> > +       if (folio_pos(folio) >= i_size)
> > +               return 0;
> 
> Function gfs2_write_jdata_page was originally introduced as
> gfs2_write_full_page in commit fd4c5748b8d3 ("gfs2: writeout truncated
> pages") to allow writing pages even when they are beyond EOF, as the
> function description documents.

Well, that was stupid of me.

> This hack was added because simply skipping journaled pages isn't
> enough on gfs2; before a journaled page can be freed, it needs to be
> marked as "revoked" in the journal. Journal recovery will then skip
> the revoked blocks, which allows them to be reused for regular,
> non-journaled data. We can end up here in contexts in which we cannot
> "revoke" pages, so instead, we write the original pages even when they
> are beyond EOF. This hack could be revisited, but it's pretty nasty
> code to pick apart.
> 
> So at least the above if needs to go for now.

Understood.  So we probably don't want to waste time zeroing the folio
if it is entirely beyond i_size, right?  Because at the moment we'd
zero some essentially random part of the folio if I just take out the
check.  Should it look like this?

        if (folio_pos(folio) < i_size &&
            i_size < folio_pos(folio) + folio_size(folio))
               folio_zero_segment(folio, offset_in_folio(folio, i_size),
                                folio_size(folio));