Commit ef425673 authored by NeilBrown's avatar NeilBrown
Browse files

md/bitmap: optimise scanning of empty bitmaps. 



A bitmap is stored as one page per 2048 bits.
If none of the bits are set, the page is not allocated.

When bitmap_get_counter finds that a page isn't allocate,
it just reports that one bit work of space isn't flagged,
rather than reporting that 2048 bits worth of space are
unflagged.
This can cause searches for flagged bits (e.g. bitmap_close_sync)
to do more work than is really necessary.

So change bitmap_get_counter (when creating) to report a number of
blocks that more accurately reports the range of the device for which
no counter currently exists.

Signed-off-by: default avatarNeilBrown <neilb@suse.de>
parent b63d7c2e

drivers/md/bitmap.c

+13 −10
Original line number Diff line number Diff line
@@ -1235,30 +1235,33 @@ __acquires(bitmap->lock) 
	unsigned long page = chunk >> PAGE_COUNTER_SHIFT;

	unsigned long pageoff = (chunk & PAGE_COUNTER_MASK) << COUNTER_BYTE_SHIFT;

	sector_t csize;

	int err;



	err = bitmap_checkpage(bitmap, page, create);



	if (bitmap_checkpage(bitmap, page, create) < 0) {

	if (bitmap->bp[page].hijacked ||

	    bitmap->bp[page].map == NULL)

		csize = ((sector_t)1) << (CHUNK_BLOCK_SHIFT(bitmap) +

					  PAGE_COUNTER_SHIFT - 1);

	else

		csize = ((sector_t)1) << (CHUNK_BLOCK_SHIFT(bitmap));

	*blocks = csize - (offset & (csize - 1));



	if (err < 0)

		return NULL;

	}



	/* now locked ... */



	if (bitmap->bp[page].hijacked) { /* hijacked pointer */

		/* should we use the first or second counter field

		 * of the hijacked pointer? */

		int hi = (pageoff > PAGE_COUNTER_MASK);

		csize = ((sector_t)1) << (CHUNK_BLOCK_SHIFT(bitmap) +

					  PAGE_COUNTER_SHIFT - 1);

		*blocks = csize - (offset & (csize - 1));

		return  &((bitmap_counter_t *)

			  &bitmap->bp[page].map)[hi];

	} else { /* page is allocated */

		csize = ((sector_t)1) << (CHUNK_BLOCK_SHIFT(bitmap));

		*blocks = csize - (offset & (csize - 1));

	} else /* page is allocated */

		return (bitmap_counter_t *)

			&(bitmap->bp[page].map[pageoff]);

}

}



int bitmap_startwrite(struct bitmap *bitmap, sector_t offset, unsigned long sectors, int behind)

{

CRA Git | Maintained and supported by SUSTech CRA and CCSE