Re: [PATCH v2] mm/pdx: Add comments throughout the codebase for pdx

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.07.2023 18:12, Alejandro Vallejo wrote:
> On Mon, Jul 10, 2023 at 09:43:34AM +0200, Jan Beulich wrote:
>> This is ...
>>>>> [snip]
>>>>> + * This is where compression becomes useful. The idea is to note that if
>>>>> + * you have several big chunks of memory sufficiently far apart you can
>>>>> + * ignore the middle part of the address because it will always contain
>>>>> + * zeroes as long as the base address is sufficiently well aligned and 
>>>>> the
>>>>> + * length of the region is much smaller than the base address.
>>>>
>>>> As per above alignment of the base address doesn't really matter.
>>> Where above?
>>
>> ... what "above" here meant.
>>
>>> As far as I understand you need enough alignment to cover the
>>> hole or you won't have zeroes to compress. Point in case:
>>>
>>>   * region1: [0x0000000000000000 -
>>>               0x00000000FFFFFFFF]
>>>
>>>   * region2: [0x0001FFFFFFFFF000 -
>>>               0x00020000FFFFFFFF]
>>>
>>> I can agree this configuration is beyond dumb and statistically unlikely to
>>> exist in the wild, but it should (IMO) still be covered by that comment.
>>
>> Right, but this isn't relevant here - in such a case no compression
>> can occur, yes, but not (just) because of missing alignment. See the
>> example I gave above (in the earlier reply) for where alignment
>> clearly doesn't matter for compression to be possible.
>>
>> Jan
> Fair enough. Then I think we can simply drop the last sentence and be done
> with it.
> 
> So the paragraph becomes:
> ```
>    * This is where compression becomes useful. The idea is to note that if
>    * you have several big chunks of memory sufficiently far apart you can
>    * ignore the middle part of the address because it will always contain
>    * zeroes.
> ```
> The details on when or how compression is possible are implicit in the
> following example anyway.
> 
> Would that, combined with v3, take care of everything?

I'm yet to look at v3. Sorry, too many things going on / pending.

Jan