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

[Adrian Hunter]

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.

> 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.

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