Re: [PATCH] design: fix computation of buffer log item bitmap size

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, Dec 23, 2021 at 11:36:15AM +0530, Chandan Babu R wrote:
> On 10 Nov 2021 at 07:28, Darrick J. Wong wrote:
> > From: Darrick J. Wong <djwong@kernel.org>
> >
> > Wengang Wang was trying to work through a buffer log item by consulting
> > the ondisk format documentation, and was confused by the formula given
> > in section 14.3.14 regarding the size of blf_data_map, aka the dirty
> > bitmap for buffer log items.  We noticed that the documentation doesn't
> > match what the kernel actually does, so let's fix this.
> >
> > Reported-by: Wengang Wang <wen.gang.wang@oracle.com>
> > Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> > ---
> >  .../journaling_log.asciidoc                        |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/design/XFS_Filesystem_Structure/journaling_log.asciidoc b/design/XFS_Filesystem_Structure/journaling_log.asciidoc
> > index 1dba56e..894d3e5 100644
> > --- a/design/XFS_Filesystem_Structure/journaling_log.asciidoc
> > +++ b/design/XFS_Filesystem_Structure/journaling_log.asciidoc
> > @@ -992,7 +992,7 @@ The size of +blf_data_map+, in 32-bit words.
> >  This variable-sized array acts as a dirty bitmap for the logged buffer.  Each
> >  1 bit represents a dirty region in the buffer, and each run of 1 bits
> >  corresponds to a subsequent log item containing the new contents of the buffer
> > -area.  Each bit represents +(blf_len * 512) / (blf_map_size * NBBY)+ bytes.
> > +area.  Each bit represents +(blf_len * 512) / (blf_map_size * sizeof(unsigned int) * NBBY)+ bytes.
> >  
> >  [[Buffer_Data_Log_Item]]
> >  === Buffer Data Log Item
> 
> The calculation looks correct. However, wouldn't it be better to mention,
> 
> "Each bit represents XFS_BLF_CHUNK (i.e. 128) bytes"
> 
> ... or some such variant involving XFS_BLF_CHUNK.

Ok.

--D

> 
> -- 
> chandan