Re: max_discard anomaly on certain Sandisk eMMC

Stephen Warren
2013-12-16T23:18:12Z

On 12/13/2013 03:43 PM, Stephen Warren wrote:
> On one of my eMMC devices, I see the following results from calling
> mmc_do_calc_max_discard() with various parameters:
>
> [ 3.057263] MMC_DISCARD_ARG max_discard 1
> [ 3.057266] MMC_ERASE_ARG max_discard 4096
> [ 3.057267] MMC_TRIM_ARG max_discard 1
>
> This causes mmc_calc_max_discard() to return 1, which makes the discard
> IOCTL extremely slow.

Further investigation shows that if I make a few hacks that essentially
revert e056a1b5b67b "mmc: queue: let host controllers specify maximum
discard timeout":

diff --git a/drivers/mmc/card/queue.c b/drivers/mmc/card/queue.c
index 357bbc54fe4b..e66af930d0e3 100644
--- a/drivers/mmc/card/queue.c
+++ b/drivers/mmc/card/queue.c
@@ -167,13 +167,15 @@ static void mmc_queue_setup_discard(struct
request_queue *q,
return;

queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, q);
- q->limits.max_discard_sectors = max_discard;
+ q->limits.max_discard_sectors = UINT_MAX;
if (card->erased_byte == 0 && !mmc_can_discard(card))
q->limits.discard_zeroes_data = 1;
q->limits.discard_granularity = card->pref_erase << 9;
/* granularity must not be greater than max. discard */
+#if 0
if (card->pref_erase > max_discard)
q->limits.discard_granularity = 0;
+#endif
if (mmc_can_secure_erase_trim(card))
queue_flag_set_unlocked(QUEUE_FLAG_SECDISCARD, q);
}

I end up with:

$ cat /sys/.../block/mmcblk1/queue# cat discard_granularity
2097152
$ cat /sys/.../block/mmcblk1/queue# cat discard_max_bytes
2199023255040
$ cat /sys/.../block/mmcblk1/queue# cat discard_zeroes_data
1

With those values, mke2fs is fast, and I validated that "blkdiscard"
works; I filled a large partition with /dev/urandom, executed
"blkdiscard" on the 4M at the start, and saw zeroes when reading the
discarded part back.

This implies that the issue is simply the operation of
mmc_calc_max_discard(), rather than the eMMC device mis-reporting its
discard abilities, doesn't it?