On Mon, Jul 16, 2012 at 02:35:18PM -0400, Mikulas Patocka wrote:
> dm-thin: fix discard support
>
> There is a bug in dm_thin regarding processing discards.
> When dm-thin receives a discard request with size equal to block size
> that is not aligned on block size boundary, io_overlaps_block returns
> true, process_discard treats this discard as a full block discard,
^^^^
> deletes the full block - the result is that some data that shouldn't be
> discarded are discarded.
Looking at io_overlaps_block(), it looks like it will return false (and
not true) for bios which are not aligned to block size boundary.
static int io_overlaps_block(struct pool *pool, struct bio *bio)
{
return !(bio->bi_sector & pool->offset_mask) &&
(bio->bi_size == (pool->sectors_per_block << SECTOR_SHIFT));
}
Hence for block which crosses block size boundary, we should be sending
discard down for partial block as per the current code and no harm should
be done?
>
> This patch sets the variable "ti->split_discard_requests", so that
> device mapper core splits discard requests on a block boundary.
>
> Consequently, a discard request that spans multiple blocks is never sent
> to dm-thin. The patch also removes some code in process_discard that
> deals with discards that span multiple blocks.
>
> Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
>
> ---
> drivers/md/dm-thin.c | 18 +++++++-----------
> 1 file changed, 7 insertions(+), 11 deletions(-)
>
> Index: linux-3.5-rc6-fast/drivers/md/dm-thin.c
> ===================================================================
> --- linux-3.5-rc6-fast.orig/drivers/md/dm-thin.c 2012-07-16 18:46:18.000000000 +0200
> +++ linux-3.5-rc6-fast/drivers/md/dm-thin.c 2012-07-16 20:07:19.000000000 +0200
> @@ -1246,17 +1246,10 @@ static void process_discard(struct thin_
> }
> } else {
> /*
> - * This path is hit if people are ignoring
> - * limits->discard_granularity. It ignores any
> - * part of the discard that is in a subsequent
> - * block.
> + * The dm makes sure that the discard doesn't span
> + * a block boundary. So we submit the discard
> + * to the appropriate block.
> */
> - sector_t offset = pool->sectors_per_block_shift >= 0 ?
> - bio->bi_sector & (pool->sectors_per_block - 1) :
> - bio->bi_sector - block * pool->sectors_per_block;
> - unsigned remaining = (pool->sectors_per_block - offset) << SECTOR_SHIFT;
> - bio->bi_size = min(bio->bi_size, remaining);
> -
So previous code will also send down partial block discard and this code
will also send down partial discard. So nothing has changed from
functionality point of view?
Thanks
Vivek
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