On Tue, 17 Jul 2012, Mike Snitzer wrote: > On Mon, Jul 16 2012 at 2:35pm -0400, > Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: > > > dm-thin: fix discard_granularity > > > > The kernel expects that limits->discard_granularity is a power of two. > > Set this limit only if we use a power of two block size. > > > > Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx> > > > > --- > > drivers/md/dm-thin.c | 3 ++- > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > 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 20:07:49.000000000 +0200 > > +++ linux-3.5-rc6-fast/drivers/md/dm-thin.c 2012-07-16 20:08:01.000000000 +0200 > > @@ -2502,7 +2502,8 @@ static void set_discard_limits(struct po > > * bios cover a block partially. A discard that spans a block boundary > > * is not sent to this target. > > */ > > - limits->discard_granularity = pool->sectors_per_block << SECTOR_SHIFT; > > + if (pool->sectors_per_block_shift >= 0) > > + limits->discard_granularity = pool->sectors_per_block << SECTOR_SHIFT; > > limits->discard_zeroes_data = pool->pf.zero_new_blocks; > > } > > Given the block layer's assumption that discard_granularity is always a > power of 2: thinp should disable discard if the thinp blocksize is a non > power of 2. So this patch isn't correct (discard support should be > disabled in pool_ctr based on the specified blocksize). discard_granularity is just a hint (and IMHO quite useless hint). The documentation says that it indicates a size of internal allocation unit that may be larger than the block size. The code doesn't use it this way - it is used in FITRIM ioctl where it specifies the minimum request size to be sent. It is also used in blkdev_issue_discard where it is used to round down the number of sectors to discard on discard_granularity boundary - this is wrong, it aligns request size on discard_granularity boundary, but it doesn't align request start on this boundary. I don't see a reason why we should disable discard because of an a rarely used hint. Mikulas > Future work should be: > 1) explore/eliminate the block layer's power of 2 constraints > 2) divorce thinp's discard limits (for discard passdown) from the pool's > blocksize > -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
