Re: [PATCH] f2fs: adjust the way of calculating nat block

[Jaegeuk Kim <jaegeuk@kernel.org>]

On 03/09, Chao Yu wrote:
> On 2017/3/9 13:37, Fan Li wrote:
> > 
> > 
> >> -----Original Message-----
> >> From: Jaegeuk Kim [mailto:jaegeuk@kernel.org]
> >> Sent: Thursday, March 09, 2017 5:56 AM
> >> To: Fan Li
> >> Cc: linux-f2fs-devel@lists.sourceforge.net
> >> Subject: Re: [f2fs-dev] [PATCH] f2fs: adjust the way of calculating nat block
> >>
> >> On 03/08, Fan Li wrote:
> >>> use a slightly simpler expression to calculate nat block with nid.
> >>>
> >>> Signed-off-by: Fan Li <fanofcode.li@samsung.com>
> >>> ---
> >>>  fs/f2fs/node.h | 4 +---
> >>>  1 file changed, 1 insertion(+), 3 deletions(-)
> >>>
> >>> diff --git a/fs/f2fs/node.h b/fs/f2fs/node.h index 2f9603f..f9fb5b8
> >>> 100644
> >>> --- a/fs/f2fs/node.h
> >>> +++ b/fs/f2fs/node.h
> >>> @@ -200,13 +200,11 @@ static inline pgoff_t current_nat_addr(struct f2fs_sb_info *sbi, nid_t start)
> >>>         struct f2fs_nm_info *nm_i = NM_I(sbi);
> >>>         pgoff_t block_off;
> >>>         pgoff_t block_addr;
> >>> -       int seg_off;
> >>>
> >>>         block_off = NAT_BLOCK_OFFSET(start);
> >>> -       seg_off = block_off >> sbi->log_blocks_per_seg;
> >>>
> >>>         block_addr = (pgoff_t)(nm_i->nat_blkaddr +
> >>> -               (seg_off << sbi->log_blocks_per_seg << 1) +
> >>> +               (block_off << 1) -
> >>
> >> NAK. This breaks the behavior.
> > 
> > OK, I'm guessing by "breaks the behavior", you mean make the code hard to understand?
> > I was concerned the same thing, but with the context, I think most people will be able to
> >  see right through it, and it saves some calculation, makes code as simple as possible, I
> > just think maybe it's worthy.
> 
> I know both calculation are right, assume:
> block_off = segment_off * 512 + off_in_segment
> a) origin: (segment_off * 512) * 2 + off_in_segment
> b) yours: 2 * (segment_off * 512 + off_in_segment) - off_in_segment

Oh, I missed '-' in the end of the line.
Let me add Chao's explanation in the patch. :)

Thanks,

> 
> Seem calculation of yours is more simple, but from my side, readability is also
> important, it's a bit hard to understand your calculation approach without comments.
> 
> Thanks,
> 
> > 
> > 
> >>
> >> Thanks,
> >>
> >>>                 (block_off & (sbi->blocks_per_seg - 1)));
> >>>
> >>>         if (f2fs_test_bit(block_off, nm_i->nat_bitmap))
> >>> --
> >>> 2.7.4
> > 
> > 
> > 
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> 
> 
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