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

Matthew Wilcox
2024-04-24T04:06:14Z

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.