Re: [PATCH] block: fix DISCARD_BARRIER requests

[Kyungmin Park <kmpark@infradead.org>]

On Fri, Jun 18, 2010 at 4:22 AM, Chris Mason <chris.mason@oracle.com> wrote:
> On Thu, Jun 17, 2010 at 06:54:53PM +0200, Christoph Hellwig wrote:
>> On Thu, Jun 17, 2010 at 10:10:18AM +0200, Jens Axboe wrote:
>> > Thanks, applied. There was a recent problem report on btrfs using
>> > discard, could possibly explain it if Chris assumed it was a full
>> > barrier.
>>
>> We actually have a much bigger issue with the DISCARD_BARRIER type.
>> If the discard request needs to get split into multiple smaller ones
>> we don't keep the queue drained atomically around them, so requests
>> could sneak inbetween them.  Depending on how the realtime discard
>> is implemented that could cause issues.  In my XFS prototype for it
>> I only deleted the extents from the tracking betree after the discard
>> request has returned, but other filesystems rely on full barrier
>> semantics of DISCARD_BARRIER this could cause real problems.
>
> btrfs needs to know that a write after the discard returns won't cross
> the discard, but beyond that we're happy with anything.

I tested btrfs on MMC with discard support and get failed. you can
find a scenario and details at btrfs mailing list.
I'll check it with this patch.
Please note that MMC don't have any queue at internal as SSD.

Thank you,
Kyungmin Park
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