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

[Vlastimil Babka]

On 08/31/2017 05:07 PM, Laura Abbott wrote:
> 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:
>>>>
>>>> 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.

How big problem is that? ZONE_MOVABLE should not contain kernel pages,
so from the kernel protection side we are OK? I expect there's another
check somewhere that the pages are not userspace, as that would be
unexpected on a wrong side of copy_to/from_user, no?

Also you can already miss some cases with the is_migrate_cma check,
because pages might be in the CMA pageblocks but not be allocated by CMA
itself - movable pages allocation can fallback here.

>> 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
>