On 10/28/2016 04:39 AM, Mikulas Patocka wrote:
On Wed, 26 Oct 2016, Bart Van Assche wrote:
On 10/26/2016 02:46 PM, Mikulas Patocka wrote:
I think the proper thing would be to move "discard_zeroes_data" flag into
the bio itself - there would be REQ_OP_DISCARD and REQ_OP_DISCARD_ZERO -
and if the device doesn't support REQ_OP_DISCARD_ZERO, it rejects the bio
and the caller is supposed to do zeroing manually.
Sorry but I don't like this proposal. I think that a much better solution
would be to pause I/O before making any changes to the discard_zeroes_data
flag.
The device mapper pauses all bios when it switches the table - but the bio
was submitted with assumption that it goes to a device withe
"discard_zeroes_data" set, then the bio is paused, device mapper table is
switched, the bio is unpaused, and now it can go to a device without
"discard_zeroes_data".
Hello Mikulas,
Sorry if I wasn't clear enough. What I meant is to wait until all
outstanding requests have finished before modifying the
discard_zeroes_data flag - the kind of operation that is performed by
e.g. blk_mq_freeze_queue(). Modifying the discard_zeroes_data flag after
a bio has been submitted and before it has completed could lead to
several classes of subtle bugs. Functions like __blkdev_issue_discard()
assume that the value of the discard_zeroes_data flag does not change
after this function has been called and before the submitted requests
completed.
Bart.
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