Re: [PATCH 1/3] mm/cma: manage the memory of the CMA area by using the ZONE_MOVABLE

[Vlastimil Babka]

On 08/31/2017 03:40 AM, Joonsoo Kim wrote:
> On Tue, Aug 29, 2017 at 11:16:18AM +0200, Vlastimil Babka wrote:
>> On 08/24/2017 08:36 AM, js1304@xxxxxxxxx wrote:
>>> From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
>>>
>>> 0. History
>>>
>>> This patchset is the follow-up of the discussion about the
>>> "Introduce ZONE_CMA (v7)" [1]. Please reference it if more information
>>> is needed.
>>>
>>
>> [...]
>>
>>>
>>> [1]: lkml.kernel.org/r/1491880640-9944-1-git-send-email-iamjoonsoo.kim@xxxxxxx
>>> [2]: https://lkml.org/lkml/2014/10/15/623
>>> [3]: http://www.spinics.net/lists/linux-mm/msg100562.html
>>>
>>> Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
>>> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
>>
>> The previous version has introduced ZONE_CMA, so I would think switching
>> to ZONE_MOVABLE is enough to drop previous reviews. Perhaps most of the
>> code involved is basically the same, though?
> 
> Yes, most of the code involved is the same. I considered to drop
> previous review tags but most of the code and concept is the same so I
> decide to keep review tags. I should mention it in cover-letter but I
> forgot to mention it. Sorry about that.
> 
>> Anyway I checked the current patch and did some basic tests with qemu,
>> so you can keep my ack.
> 
> Thanks!
> 
>>
>> BTW, if we dropped NR_FREE_CMA_PAGES, could we also drop MIGRATE_CMA and
>> related hooks? Is that counter really that useful as it works right now?
>> It will decrease both by CMA allocations (which has to be explicitly
>> freed) and by movable allocations (which can be migrated). What if only
>> CMA alloc/release touched it?
> 
> I think that NR_FREE_CMA_PAGES would not be as useful as previous. We
> can remove it.
> 
> However, removing MIGRATE_CMA has a problem. There is an usecase to
> check if the page comes from the CMA area or not. See
> check_page_span() in mm/usercopy.c. I can implement it differently by
> iterating whole CMA area and finding the match, but I'm not sure it's
> performance effect. I guess that it would be marginal.

+CC Kees Cook

Hmm, seems like this check is to make sure we don't copy from/to parts
of kernel memory we're not supposed to? Then I believe checking that
pages are in ZONE_MOVABLE should then give the same guarantees as
MIGRATE_CMA.

BTW the comment says "Reject if range is entirely either Reserved or
CMA" but the code does the opposite thing. I assume the comment is wrong?

> Anyway, I'd like not to cause any side-effect now. After patches are
> settle down on mainline, I will try to remove them as you suggested.
> 
> Thanks.
> 
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