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Subject: Re: [PATCH v4 6/7] dm: Directly disable max_allocate_sectors for now
From:   Kirill Tkhai <ktkhai@virtuozzo.com>
To:     Mike Snitzer <snitzer@redhat.com>
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Date:   Tue, 21 Jan 2020 16:33:50 +0300
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On 21.01.2020 15:36, Kirill Tkhai wrote:
> On 21.01.2020 15:24, Mike Snitzer wrote:
>> On Tue, Jan 21 2020 at  5:42am -0500,
>> Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
>>
>>> Since dm inherits limits from underlining block devices,
>>> this patch directly disables max_allocate_sectors for dm
>>> till full allocation support is implemented.
>>>
>>> This prevents high-level primitives (generic_make_request_checks(),
>>> __blkdev_issue_write_zeroes(), ...) from sending REQ_ALLOCATE
>>> requests.
>>>
>>> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
>>> ---
>>>  drivers/md/dm-table.c |    2 ++
>>>  drivers/md/md.h       |    1 +
>>>  2 files changed, 3 insertions(+)
>>
>> You're mixing DM and MD changes in the same patch.
>>
>> But I'm wondering if it might be best to set this default for stacking
>> devices in blk_set_stacking_limits()?
>>
>> And then it is up to each stacking driver to override as needed.
> 
> Hm. Sound like a good idea. This "lim->max_allocate_sectors = 0" in blk_set_stacking_limits()
> should work for dm's dm_calculate_queue_limits(), since it calls blk_stack_limits(), which is:
> 
> 	t->max_allocate_sectors = min(t->max_allocate_sectors,
> 				      b->max_allocate_sectors);
> 
> Could you please tell is this fix is also enough for md?

It looks like it's enough since queue defaults are set in md_alloc()->blk_set_stacking_limits().
In case of we set "max_allocate_sectors = 0", in further it can be changed only manually,
but nobody does this.