On 9/23/13 10:44 AM, David Sterba wrote:
> On Mon, Sep 23, 2013 at 10:08:08AM -0500, Eric Sandeen wrote:
>> On 9/20/13 11:42 AM, David Sterba wrote:
>>> The message about trim was printed unconditionally, we should check if
>>> trim is supported at all.
>>
>> Good idea, but I wonder if there's any risk that discard(0,0) will ever
>> be optimized away on the kernel side & pass unconditionally?
>
> I hope the checks in blkdev_issue_discard() stay in the order as of now:
>
> 40 int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
> 41 sector_t nr_sects, gfp_t gfp_mask, unsigned long flags)
> 42 {
> ...
> 52
> 53 if (!q)
> 54 return -ENXIO;
> 55
> 56 if (!blk_queue_discard(q))
> 57 return -EOPNOTSUPP;
>
> here it returns no matter what the arguments are, setting length to 0 is
> just cautious.
>
> 59 /* Zero-sector (unknown) and one-sector granularities are the same. */
> 60 granularity = max(q->limits.discard_granularity >> 9, 1U);
> 61 alignment = bdev_discard_alignment(bdev) >> 9;
> 62 alignment = sector_div(alignment, granularity);
> 63
>
>> I was thinking we could get this from blkid, but maybe not.
>
> Possibly yes, with other information like rotational etc.
>
> Alternatively,
>
> /sys/block/sdx/queue/dicard_granularity > 0 means that the device
> supports discard, but that's imo even more fragile than the direct
> call to discard.
Perhaps; and I don't think libblkid gives us easy access to that anyway,
at least I didn't see it on a quick look.
So yeah, I think it's fine as you sent it; it doesn't actually change
behavior anyway other than the printf.
Thanks,
-Eric
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