Re: [PATCH v2 7/8] mm: drop page_index/page_file_offset and convert swap helpers to use folio

[Matthew Wilcox <willy@infradead.org>]

On Wed, Apr 24, 2024 at 10:17:04AM +0800, Huang, Ying wrote:
> Kairui Song <ryncsn@gmail.com> writes:
> >  static inline loff_t folio_file_pos(struct folio *folio)
> >  {
> > -	return page_file_offset(&folio->page);
> > +	if (unlikely(folio_test_swapcache(folio)))
> > +		return __folio_swap_dev_pos(folio);
> > +	return ((loff_t)folio->index << PAGE_SHIFT);
> 
> This still looks confusing for me.  The function returns the byte
> position of the folio in its file.  But we returns the swap device
> position of the folio.
> 
> Tried to search folio_file_pos() usage.  The 2 usage in page_io.c is
> swap specific, we can use swap_dev_pos() directly.
> 
> There are also other file system users (NFS and AFS) of
> folio_file_pos(), I don't know why they need to work with swap
> cache. Cced file system maintainers for help.

Time for a history lesson!

In d56b4ddf7781 (2012) we introduced page_file_index() and
page_file_mapping() to support swap-over-NFS.  Writes to the swapfile went
through ->direct_IO but reads went through ->readpage.  So NFS was changed
to remove direct references to page->mapping and page->index because
those aren't right for anon pages (or shmem pages being swapped out).

In e1209d3a7a67 (2022), we stopped using ->readpage in favour of using
->swap_rw.  Now we don't need to use page_file_*(); we get the swap_file
and ki_pos directly in the swap_iocb.  But there are still relics in NFS
that nobody has dared rip out.  And there are all the copy-and-pasted
filesystems that use page_file_* because they don't know any better.

We should delete page_file_*() and folio_file_*().  They shouldn't be
needed any more.