linux-kernel - Re: [PATCH v3 02/12] nilfs2: drop usage of page

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Message-ID: <Zi_3OxP6xKjBWBLO@casper.infradead.org>
Date: Mon, 29 Apr 2024 20:38:35 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Ryusuke Konishi <konishi.ryusuke@...il.com>
Cc: Kairui Song <ryncsn@...il.com>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Huang, Ying" <ying.huang@...el.com>, 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-nilfs@...r.kernel.org
Subject: Re: [PATCH v3 02/12] nilfs2: drop usage of page_index

On Tue, Apr 30, 2024 at 04:28:41AM +0900, Ryusuke Konishi wrote:
> On Tue, Apr 30, 2024 at 4:22 AM Kairui Song <ryncsn@...il.com> wrote:
> >
> > On Tue, Apr 30, 2024 at 3:14 AM Matthew Wilcox <willy@...radead.org> wrote:
> > >
> > > On Tue, Apr 30, 2024 at 03:04:50AM +0800, Kairui Song wrote:
> > > > From: Kairui Song <kasong@...cent.com>
> > > >
> > > > page_index is only for mixed usage of page cache and swap cache, for
> > > > pure page cache usage, the caller can just use page->index instead.
> > > >
> > > > It can't be a swap cache page here (being part of buffer head),
> > > > so just drop it, also convert it to use folio.
> > > >
> > > > Signed-off-by: Kairui Song <kasong@...cent.com>
> > > > Cc: Ryusuke Konishi <konishi.ryusuke@...il.com>
> > > > Cc: linux-nilfs@...r.kernel.org
> > > > ---
> > > >  fs/nilfs2/bmap.c | 5 ++---
> > > >  1 file changed, 2 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/fs/nilfs2/bmap.c b/fs/nilfs2/bmap.c
> > > > index 383f0afa2cea..f4e5df0cd720 100644
> > > > --- a/fs/nilfs2/bmap.c
> > > > +++ b/fs/nilfs2/bmap.c
> > > > @@ -453,9 +453,8 @@ __u64 nilfs_bmap_data_get_key(const struct nilfs_bmap *bmap,
> > > >       struct buffer_head *pbh;
> > > >       __u64 key;
> > > >
> > > > -     key = page_index(bh->b_page) << (PAGE_SHIFT -
> > > > -                                      bmap->b_inode->i_blkbits);
> > > > -     for (pbh = page_buffers(bh->b_page); pbh != bh; pbh = pbh->b_this_page)
> > > > +     key = bh->b_folio->index << (PAGE_SHIFT - bmap->b_inode->i_blkbits);
> > > > +     for (pbh = folio_buffers(bh->b_folio); pbh != bh; pbh = pbh->b_this_page)
> > > >               key++;
> > > >
> > > >       return key;
> > >
> > > Why isn't this entire function simply:
> > >
> > >         return bh->b_blocknr;
> > >
> >
> > Nice idea, I didn't plan for extra clean up and test for fs code, but
> > this might be OK to have, will check it.
> 
> Wait a minute.
> 
> This function returns a key that corresponds to the cache offset of
> the data block, not the disk block number.
> 
> Why is returning to bh->b_blocknr an alternative ?
> Am I missing something?

Sorry, I forgot how b_blocknr was used.  What I meant was:

	u64 key = bh->b_folio->index << (PAGE_SHIFT - bmap->b_inode->i_blkbits);

	return key + bh_offset(bh) >> bmap->b_inode->i_blkbits;

The point is to get rid of the loop.  We could simplify this (and make
it ready for bs>PS) by doing:

	loff_t pos = folio_pos(bh->b_folio) + bh_offset(bh);
	return pos >> bmap->b_inode->i_blkbits;