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Date: Fri, 7 Aug 2015 22:17:43 -0700
Message-ID: <CAF1ivSYAiRqG1RhxiHE+cfogB_VKyDDs0aE7YbtZ2o6UeGhZcQ@mail.gmail.com>
Subject: Re: [PATCH v5 01/11] block: make generic_make_request handle
 arbitrarily sized bios
From: Ming Lin <mlin@kernel.org>
To: Kent Overstreet <kent.overstreet@gmail.com>
Cc: Christoph Hellwig <hch@lst.de>, Mike Snitzer <snitzer@redhat.com>,
        lkml <linux-kernel@vger.kernel.org>, Jens Axboe <axboe@kernel.dk>,
        Dongsu Park <dpark@posteo.net>, Christoph Hellwig <hch@infradead.org>,
        Al Viro <viro@zeniv.linux.org.uk>, Ming Lei <ming.lei@canonical.com>,
        Neil Brown <neilb@suse.de>, Alasdair Kergon <agk@redhat.com>,
        dm-devel@redhat.com, Lars Ellenberg <drbd-dev@lists.linbit.com>,
        drbd-user@lists.linbit.com, Jiri Kosina <jkosina@suse.cz>,
        Geoff Levand <geoff@infradead.org>, Jim Paris <jim@jtan.com>,
        Philip Kelleher <pjk1939@linux.vnet.ibm.com>,
        Minchan Kim <minchan@kernel.org>, Nitin Gupta <ngupta@vflare.org>,
        Oleg Drokin <oleg.drokin@intel.com>,
        Andreas Dilger <andreas.dilger@intel.com>,
        Ming Lin <ming.l@ssi.samsung.com>
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On Fri, Aug 7, 2015 at 5:30 PM, Kent Overstreet
<kent.overstreet@gmail.com> wrote:
> On Fri, Aug 07, 2015 at 04:40:06PM -0700, Ming Lin wrote:
>>
>> On Fri, 2015-08-07 at 09:30 +0200, Christoph Hellwig wrote:
>> > I'm for solution 3:
>> >
>> >  - keep blk_bio_{discard,write_same}_split, but ensure we never built
>> >    a > 4GB bio in blkdev_issue_{discard,write_same}.
>>
>> This has problem as I mentioned in solution 1.
>> We need to also make sure max discard size is of proper granularity.
>> See below example.
>>
>>       4G: 8388608 sectors
>> UINT_MAX: 8388607 sectors
>>
>> dm-thinp block size = default discard granularity = 128 sectors
>>
>> blkdev_issue_discard(sector=0, nr_sectors=8388608)
>>
>> 1. Only ensure bi_size not overflow
>>
>> It doesn't work.
>>
>> [start_sector, end_sector]
>> [0, 8388607]
>>     [0, 8388606], then dm-thinp splits it to 2 bios
>>         [0, 8388479]
>>         [8388480, 8388606] ---> this has problem in process_discard_bio(),
>>                                 because the discard size(7 sectors) covers less than a block(128 sectors)
>>     [8388607, 8388607] ---> same problem
>>
>> 2. Ensure bi_size not overflow and max discard size is of proper granularity
>
> Ideally we'd get upper layers out of the business of knowing about the queue
> limits at all - that was the point of the patch series, after all.
>
> Instead of using UINT_MAX, would it work to just make the max 1 << 31 sectors?'

1 << 31 = 2G bytes = 0x400000 sectors.

Yes, that works as long as it's multiple of granularity.
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