Weirdness with discard cmd and get log pages

Keith Busch keith.busch at
Fri Oct 14 10:44:21 PDT 2016

On Thu, Oct 13, 2016 at 07:18:09PM -0400, Keith Busch wrote:
> On Thu, Oct 13, 2016 at 11:18:43AM -0700, Nisha Miller wrote:
> > Yes, that is what I noticed too. I used the nvme-cli command like this:
> > 
> > nvme dsm /dev/nvme0n1 -a 0,0,0,0 --blocks=4,5,6,7 --slbs=100,200,300,400 --ad
> > 
> > This turns up as nvme_user_cmd in the driver, which calls
> > nvme_map_user_pages to setup the SG list.
> Okay, that's what I use too. I'm not observing any issues on a 4.8 kernel
> or back to 4.4 either. I've not tested 3.19 though, and the mechanism
> it uses to map user buffers is completely different. Could you verify if
> your observation exists in a more current stable release?

Just for reference, this is how I've verified 64 ranges. My device
deterministically returns 0 on any deallocated block, and is formatted
with 512b LBAs. 

  # create a random 1MB file
  dd if=/dev/urandom of=~/ bs=1M count=1

  # write it to the device
  dd if=~/ of=/dev/nvme0n1 oflag=direct

  # read it back out
  dd if=/dev/nvme0n1 of=~/rand.1M.out bs=1M count=1 iflag=direct

  # compare the two to verify they're the same
  diff ~/ ~/rand.1M.out

  # write a bunch of 0-filled 8k holes in the original file
  for i in $(seq 0 2 127); do dd if=/dev/zero of=~/ bs=8k seek=$i conv=notrunc count=1 2> /dev/null; done

  # deallocate the exact same ranges as the file's new 0-filled holes 
  nvme dsm /dev/nvme0n1 -d --slbs=`seq 0 32 2016 | tr "\n" "," | sed "s/,$//g"` --blocks=`printf "16,%0.s" {0..63} | sed "s/,$//g"`

  # read the file from the device
  dd if=/dev/nvme0n1 of=~/rand.1M.out bs=1M count=1 iflag=direct

  # verify the contents are still the same
  diff ~/ ~/rand.1M.out

Works for me.

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