Re: [PATCH v2] dm-io: deal with wandering queue limits when handling REQ_DISCARD and REQ_WRITE_SAME

[Mike Snitzer <snitzer@redhat.com>]

On Fri, Feb 27 2015 at  2:09pm -0500,
Mikulas Patocka <mpatocka@redhat.com> wrote:

> 
> 
> On Fri, 27 Feb 2015, Darrick J. Wong wrote:
> 
> > Since it's apparently possible that the queue limits for discard and
> > write same can change while the upper level command is being sliced
> > and diced, fix up both of them (a) to reject IO if the special command
> > is unsupported at the start of the function and (b) read the limits
> > once and let the commands error out on their own if the status happens
> > to change.
> > 
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> 
> > +	unsigned int special_cmd_max_sectors;
> > +
> > +	/* Reject unsupported discard and write same requests */
> > +	if (rw & REQ_DISCARD)
> > +		special_cmd_max_sectors = q->limits.max_discard_sectors;
> > +	else if (rw & REQ_WRITE_SAME)
> > +		special_cmd_max_sectors = q->limits.max_write_same_sectors;
> > +	if ((rw & (REQ_DISCARD | REQ_WRITE_SAME)) &&
> > +	    special_cmd_max_sectors == 0) {
> 
> That results in uninitialized variable warning (although the warning is 
> false positive). We need the macro uninitialized_var to suppress the 
> warning.
> 
> It's better to use ACCESS_ONCE on variables that may be changing so that 
> the compiler doesn't load them multiple times.

I dropped the use of ACCESS_ONCE.  We access queue_limits all over block
related code.  If the performance is quantifiable then all accesses
should be updated.  Until then, I'm maintaining status-quo.

Slightly tweaked commit is staged for 4.0 here:
https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-for-4.0&id=e5db29806b99ce2b2640d2e4d4fcb983cea115c5