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

[David Jander]

On Mon, 01 Jun 2015 13:36:45 +0300
Adrian Hunter <adrian.hunter@xxxxxxxxx> 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@xxxxxxxxxxx>
> > ---
> >  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?
Has anybody even started to look into this?

Best regards,

-- 
David Jander
Protonic Holland.
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