Re: [PATCH 1/6] mm/memblock: reduce the two round insertion of memblock_add_range()

[Wei Yang <richard.weiyang@gmail.com>]

On Wed, Apr 24, 2024 at 04:15:30PM +0300, Mike Rapoport wrote:
>On Mon, Apr 22, 2024 at 02:55:00AM +0000, Wei Yang wrote:
>> On Mon, Apr 15, 2024 at 06:17:56PM +0300, Mike Rapoport wrote:
>> >On Sun, Apr 14, 2024 at 12:45:26AM +0000, Wei Yang wrote:
>> >> Current memblock_add_range() does the insertion with two round
>> >> iteration.
>> >> 
>> >> First round does the calculation of new region required, and second
>> >> round does the actual insertion. Between them, if current max can't meet
>> >> new region requirement, it is expanded.
>> >> 
>> >> The worst case is:
>> >> 
>> >> 1. cnt == max
>> >> 2. new range overlaps all existing region
>> >> 
>> >> This means the final cnt should be (2 * max + 1). Since we always double
>> >> the array size, this means we only need to double the array twice at the
>> >> worst case, which is fairly rare. For other cases, only once array
>> >> double is enough.
>> >> 
>> >> Let's double the array immediately when there is no room for new region.
>> >> This simplify the code a little.
>> >
>> >Very similar patch was posted a while ago:
>> >https://lore.kernel.org/all/20221025070943.363578-1-yajun.deng@linux.dev
>> >
>> >and it caused boot regression:
>> >https://lore.kernel.org/linux-mm/Y2oLYB7Tu7J91tVm@linux.ibm.com
>> >
>> 
>> Got the reason for the error.
>> 
>> When we add range to reserved and need double array, following call happends:
>> 
>> memblock_add_range()
>>     idx <- where we want to insert new range
>>     memblock_double_array()
>>         find available range for new regions
>> 	memblock_reserve() available range
>>     memblock_insert_region() at idx
>> 
>> The error case happens when find available range below what we want to add,
>> the idx may change after memblock_reserve().
>> 
>> The following change looks could fix it.
>
>I don't think the micro-optimization of the second loop in
>memblock_add_range() worth the churn and potential bugs.
> 

Sure, would drop this one.

-- 
Wei Yang
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