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

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Date: Wed, 24 Apr 2024 14:45:26 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Kairui Song <ryncsn@...il.com>,  linux-mm@...ck.org,  Kairui Song
 <kasong@...cent.com>,  Andrew Morton <akpm@...ux-foundation.org>,  Chris
 Li <chrisl@...nel.org>,  Barry Song <v-songbaohua@...o.com>,  Ryan Roberts
 <ryan.roberts@....com>,  Neil Brown <neilb@...e.de>,  Minchan Kim
 <minchan@...nel.org>,  Hugh Dickins <hughd@...gle.com>,  David Hildenbrand
 <david@...hat.com>,  Yosry Ahmed <yosryahmed@...gle.com>,
  linux-fsdevel@...r.kernel.org,  linux-kernel@...r.kernel.org,
  linux-nfs@...r.kernel.org,  Trond Myklebust
 <trond.myklebust@...merspace.com>,  Anna Schumaker <anna@...nel.org>,
  linux-afs@...ts.infradead.org,  David Howells <dhowells@...hat.com>,
  Marc Dionne <marc.dionne@...istor.com>
Subject: Re: [PATCH v2 7/8] mm: drop page_index/page_file_offset and convert
 swap helpers to use folio

Hi, Matthew,

Matthew Wilcox <willy@...radead.org> writes:

> On Wed, Apr 24, 2024 at 10:17:04AM +0800, Huang, Ying wrote:
>> Kairui Song <ryncsn@...il.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 a lot for your detailed explanation!  Yes, this will simplify the
semantics and improve the readability of the corresponding code.

--
Best Regards,
Huang, Ying