Hi Neil,

NeilBrown wrote:

quoted

On Tue, 17 Mar 2015 10:40:30 +0800 jgq516@gmail.com wrote:

  

quoted

From: Guoqing Jiang <redacted>

The bm_blocks is modified by commit fe60ce (md/bitmap: use
sector_div for sector_t divisions), but it makes bm_blocks
has different value which is changed from like "a/b" to "a%b",
need to correct this to make sure cluster-md still works.
    

One of us is confused here.

This code is trying to find the start of the bitmap relevant to this host in
a table of multiple bitmaps.  So it first needs to find out the size of each
bitmap.  It then multiples the size by the index number of this host to get
an offset.

  

Thanks for detailed description, it really helps. I quoted related lines
from bitmap.c.

 574                 sector_t bm_blocks;
 575                 sector_t resync_sectors =
bitmap->mddev->resync_max_sectors; 
 576
 577                 bm_blocks = sector_div(resync_sectors,
 578                                       
bitmap->mddev->bitmap_info.chunksize >> 9);   
 579                 bm_blocks = bm_blocks << 3;
 580                 bm_blocks = DIV_ROUND_UP_SECTOR_T(bm_blocks, 4096);
 581                 bitmap->mddev->bitmap_info.offset +=
bitmap->cluster_slot * (bm_blocks << 3);

quoted

So it take the total number of sectors (resync_max_sectors), divides by the
chunksize (in sectors) to get a number of chunks.  This is the number of bits.

  

L577 is supposed to do above job.

quoted

Then it should div-round-up by 8 to get a number of bytes.
  

I guess what you mean is about L579, while it used "<<3" rather than
">>3" now.

quoted

Then div-round-up by 4096 to get number of 4-K blocks, because the bitmaps
are always 4K aligned.
  

L580 did the job.

quoted

Then this number is multiplied by 8 (or shifted by 3) to get a number of
sectors to add to the start of the table.
  

L581 is for this, right? Is the shifted by 3 is to match the bitmap
format for each
nodes? Seems the relationship between slot and the bitmap region of the node
is like n <-----> [8*nK, 8*(n+1)K]. How about the following changes?

diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
index 501f83f..b2a241b 100644
--- a/drivers/md/bitmap.c
+++ b/drivers/md/bitmap.c

@@ -571,12 +571,10 @@ static int bitmap_read_sb(struct bitmap *bitmap)
 re_read:
        /* If cluster_slot is set, the cluster is setup */
        if (bitmap->cluster_slot >= 0) {
-               sector_t bm_blocks;
-               sector_t resync_sectors = bitmap->mddev->resync_max_sectors;
+               sector_t bm_blocks = bitmap->mddev->resync_max_sectors;
 
-               bm_blocks = sector_div(resync_sectors,
-                                     

bitmap->mddev->bitmap_info.chunksize >> 9);
-               bm_blocks = bm_blocks << 3;
+               sector_div(bm_blocks,
bitmap->mddev->bitmap_info.chunksize >> 9);