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

[Laura Abbott <labbott@redhat.com>]

On 08/31/2017 07:32 AM, Vlastimil Babka wrote:
> 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@gmail.com wrote:
>>>> From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>>>>
>>>> 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@lge.com
>>>> [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@linux.vnet.ibm.com>
>>>> Acked-by: Vlastimil Babka <vbabka@suse.cz>
>>>
>>> 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.
> 

The check is to make sure we are copying only to a single page unless
that page is allocated with __GFP_COMP. CMA needs extra checks since
its allocations have nothing to do with compound page. Checking
ZONE_MOVABLE might cause us to miss some cases of copying to vanilla
ZONE_MOVABLE pages.

> 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?
> 

Yes, I think that needs clarification.

Thanks,
Laura

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