Hi, Matthew,
Matthew Wilcox <willy@xxxxxxxxxxxxx> writes:
> On Wed, Apr 24, 2024 at 10:17:04AM +0800, Huang, Ying wrote:
>> Kairui Song <ryncsn@xxxxxxxxx> 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 a lot for your detailed explanation! Yes, this will simplify the
semantics and improve the readability of the corresponding code.
--
Best Regards,
Huang, Ying
