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

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Date: Tue, 30 Apr 2024 23:27:13 +0800
From: Kairui Song <ryncsn@...il.com>
To: Ryusuke Konishi <konishi.ryusuke@...il.com>
Cc: Matthew Wilcox <willy@...radead.org>, 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 4:01 AM Ryusuke Konishi
<konishi.ryusuke@...il.com> wrote:
>
> On Tue, Apr 30, 2024 at 4:38 AM Matthew Wilcox wrote:
> >
> > 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;
>
> I see, I understand the idea that it would be better to eliminate the loop.
>
> The above conversion looks fine.
> What are you going to do, Kairui ?

Hi, I'd like to remove the loop as Matthew suggested, that will make
the code cleaner.
I'm not very familiar with this part so I'll check related code first
for double check, won't take long though.

> Thanks,
> Ryusuke Konishi