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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Message-ID: <ZiiFHTwgu8FGio1k@casper.infradead.org>
Date: Wed, 24 Apr 2024 05:05:49 +0100
From: Matthew Wilcox <willy@...radead.org>
To: "Huang, Ying" <ying.huang@...el.com>
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

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.