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

Kairui Song
2024-04-24T08:49:05Z

On Wed, Apr 24, 2024 at 12:06 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> 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.

Thanks for the explanation! I'll update the series, and just delete
paeg_file_offset and folio_file_pos with more auditing, to make the
code cleaner. Should I add a suggest-by for the removal?