Re: [PATCH 1/4] ext4: Convert move_extent_per_page() to use folios

[Vishal Moola <vishal.moola@gmail.com>]

On Wed, Nov 16, 2022 at 12:09 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Tue, Nov 15, 2022 at 06:10:08PM -0800, Vishal Moola (Oracle) wrote:
> >  {
> >       struct inode *orig_inode = file_inode(o_filp);
> >       struct page *pagep[2] = {NULL, NULL};
> > +     struct folio *folio[2] = {NULL, NULL};
>
> I have a feeling that mext_page_double_lock() should also be converted
> to use folios.  But this makes me nervous:
>
>         int blocks_per_page = PAGE_SIZE >> orig_inode->i_blkbits;
>
> and I'm not sure what will happen if one or both of the orig_page
> and donor_page is large -- possibly different sizes of large.
>
> Obviously ext4 doesn't allow large folios today, but it would be good to
> get some reasoning about why this isn't laying a trap for later (or at
> least assertions that neither folio is large so that there's an obvious
> scream instead of silent data corruption).

I had thought once mext_page_mkuptodate() and block_commit_write()
were converted to folios, large folios wouldn't be a problem. I hadn't
considered
that the folios may be of different sizes. I can add assertions about both
folios being large and the same size in v2.