Re: [PATCH] mmc: core: Fix off-by-one error in mmc_do_calc_max_discard()

[David Jander <david@protonic.nl>]

On Mon, 01 Jun 2015 15:38:51 +0300
Adrian Hunter <adrian.hunter@intel.com> wrote:

> On 01/06/15 15:30, David Jander wrote:
> > On Mon, 01 Jun 2015 14:50:47 +0300
> > Adrian Hunter <adrian.hunter@intel.com> wrote:
> > 
> >> On 01/06/15 14:32, David Jander wrote:
> >>> On Mon, 01 Jun 2015 13:36:45 +0300
> >>> Adrian Hunter <adrian.hunter@intel.com> wrote:
> >>>
> >>>> On 01/06/15 12:20, David Jander wrote:
> >>>>> qty is the maximum number of discard that _do_ fit in the timeout, not
> >>>>> the first amount that does _not_ fit anymore.
> >>>>> This seemingly harmless error has a very severe performance impact when
> >>>>> the timeout value is enough for only 1 erase group.
> >>>>>
> >>>>> Signed-off-by: David Jander <david@protonic.nl>
> >>>>> ---
> >>>>>  drivers/mmc/core/core.c | 7 ++-----
> >>>>>  1 file changed, 2 insertions(+), 5 deletions(-)
> >>>>>
> >>>>> diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
> >>>>> index 92e7671..1f9573b 100644
> >>>>> --- a/drivers/mmc/core/core.c
> >>>>> +++ b/drivers/mmc/core/core.c
> >>>>> @@ -2234,16 +2234,13 @@ static unsigned int
> >>>>> mmc_do_calc_max_discard(struct mmc_card *card, if (!qty)
> >>>>>  		return 0;
> >>>>>  
> >>>>> -	if (qty == 1)
> >>>>> -		return 1;
> >>>>> -
> >>>>>  	/* Convert qty to sectors */
> >>>>>  	if (card->erase_shift)
> >>>>> -		max_discard = --qty << card->erase_shift;
> >>>>> +		max_discard = qty << card->erase_shift;
> >>>>>  	else if (mmc_card_sd(card))
> >>>>>  		max_discard = qty;
> >>>>>  	else
> >>>>> -		max_discard = --qty * card->erase_size;
> >>>>> +		max_discard = qty * card->erase_size;
> >>>>>  
> >>>>>  	return max_discard;
> >>>>>  }
> >>>>>
> >>>>
> >>>> This keeps coming up but there is more to it than that.  See here:
> >>>>
> >>>> 	http://marc.info/?l=linux-mmc&m=142504164427546
> >>>>
> >>>
> >>> Thanks for the link. I think it is time to put a comment on that piece of
> >>> code to clarify this.
> >>> Also, this code badly needs optimizing. I happen to have one of those
> >>> unfortunate cases, where the maximum timeout of the MMC controller
> >>> (Freescale i.MX6 uSDHCI) is 5.4 seconds, and the eMMC device (Micron 16GB
> >>> eMMC) TRIM_MULT is 15 (4.5 seconds). As a result
> >>> mmc_do_calc_max_discard() returns 1 and mkfs.ext4 takes several hours!!
> >>> I think it is pretty clear that this is unacceptable and needs to be
> >>> fixed. AFAICS, the "correct fix" for this would implicate that discard
> >>> knows about the erase-group boundaries... something that could reach
> >>> into the block-layer even... right?
> >>
> >> Not necessarily. You could regard the "can only do 1 erase block at a
> >> time" case as special, flag it, and in that case have mmc_erase() split
> >> along erase block boundaries and call mmc_do_erase() multiple times. Then
> >> you could set max_discard to something arbitrarily bigger.
> > 
> > Right. I was just looking at mmc_erase() and thought about splitting the
> > erase at the next boundary if it was not aligned. That way my patch could
> > be used in every case, since we would ensure that mmc_do_erase() will
> > always start erase-group aligned. Would you agree to such a solution?
> 
> Why would people who don't have your problem want their erase performance
> potentially degraded by unnecessary splitting.

This penalty would exist only when erasing a small amount of sectors. If we
approach the timeout limit, this penalty is canceled-out by the gain of being
able to erase double the amount of sectors in one operation. I have no idea
what the typical workload of this function will be, so I take your hint and
treat the "can only do 1 erase block at a time" case as special.

>[...]
> >>> Has anybody even started to look into this?
> >>
> >> Ulf was looking at supporting R1 response instead of R1b response from the
> >> erase command and using a software timeout instead of the host
> >> controller's hardware timeout.
> > 
> > That would also be an option, specially if the TRIM_MULT becomes larger
> > than what the controller can handle!
> > @Ulf: How far are you with this?

Still wonder about this case, though...

Best regards,

-- 
David Jander
Protonic Holland.