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

[David Jander]

On Mon, 01 Jun 2015 14:50:47 +0300
Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:

> On 01/06/15 14:32, David Jander wrote:
> > 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?
> 
> 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?
Just to be clear, I propose:

 1. mmc_do_calc_max_discard() assumes erase-group-aligned discards, and thus
 returns "qty * card->erase_size" instead of "--qty * card->erase_size".

 2. mmc_erase() always splits off the first chunk that is not
 erase-group-aligned and may thus call mmc_do_erase() twice in succession if
 necessary.

No special treatment needed.

> > 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?

Best regards,

-- 
David Jander
Protonic Holland.

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