linux-ext4 - Re: do not disable ext4 discards on first discard failure? [was: Re: dm snapshot: ignore discards issued to the snapshot-origin target]

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Date:	Wed, 04 May 2011 12:50:57 -0400
From:	"Martin K. Petersen" <martin.petersen@...cle.com>
To:	Mike Snitzer <snitzer@...hat.com>
Cc:	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Lukas Czerner <lczerner@...hat.com>, sandeen@...hat.com,
	Christoph Hellwig <hch@...radead.org>,
	device-mapper development <dm-devel@...hat.com>,
	DarkNovaNick@...il.com, linux-lvm@...hat.com,
	linux-ext4@...r.kernel.org, Alasdair G Kergon <agk@...hat.com>
Subject: Re: do not disable ext4 discards on first discard failure? [was: Re: dm snapshot: ignore discards issued to the snapshot-origin target]

>>>>> "Mike" == Mike Snitzer <snitzer@...hat.com> writes:

Mike> lim->discard_zeroes_data = -1; was suspect to me too.
Mike> But why default to 1 here?

Because otherwise DM would default to having dzd to "unsupported",
meaning the feature would never be turned on regardless of the bottom
device capabilities.

The old approach used the -1 value to indicate "has not been set". That
was only really intended as a value for the stacking drivers, not for
the LLDs. It was a bit of a hack and I'd rather deal with dzd the same
way as we do with clustering.

>> @@ -166,6 +166,7 @@ void blk_queue_make_request(struct request_queue
>> *q, make_request_fn *mfn)
>> 
>> blk_set_default_limits(&q->limits);
>> blk_queue_max_hw_sectors(q, BLK_SAFE_MAX_SECTORS);
>> + q->limits.discard_zeroes_data = 0;
>> 
>> /*
>> * by default assume old behaviour and bounce for any highmem page

Mike> Only to then reset to 0 here?  Shouldn't we default to 0 and only
Mike> set to 1 where applicable (e.g. sd_config_discard)?

My first approach was to set it in dm-table.c before stacking. But I
thought it was icky to have the stacking driver ask for defaults and
then have to tweak them for things to work correctly.

The other option is to have blk_set_default_stacking_limits(). Or we
could add a flag to blk_set_default_limits to indicate whether this is a
LLD or a stacking driver.

We already special-case BLK_SAFE_MAX_SECTORS when setting the request
function. And that's the only non-stacking user of the default limits
call. So that's why I disabled dzd there. Since this is a stable bugfix
I also wanted to keep it small and simple. But I'm totally open to
suggestions.

-- 
Martin K. Petersen	Oracle Linux Engineering
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