Re: [PATCH v3] mm/hugetlb: avoid hardcoding while checking if cma is enable

[Mike Kravetz]

On 7/9/20 4:45 PM, Song Bao Hua (Barry Song) wrote:
> 
> 
>> -----Original Message-----
>> From: Mike Kravetz [mailto:mike.kravetz@xxxxxxxxxx]
>> Sent: Friday, July 10, 2020 6:58 AM
>> To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>; Roman
>> Gushchin <guro@xxxxxx>
>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>; linux-mm@xxxxxxxxx;
>> linux-kernel@xxxxxxxxxxxxxxx; Linuxarm <linuxarm@xxxxxxxxxx>; Jonathan
>> Cameron <jonathan.cameron@xxxxxxxxxx>
>> Subject: Re: [PATCH v3] mm/hugetlb: avoid hardcoding while checking if cma
>> is enable
>>
>> Looks like this produced a warning in linux-next.  I suspect it is due to the
>> combination CONFIG_HUGETLB_PAGE && !CONFIG_CMA.
>>
>> Instead of adding the routine hugetlb_cma_enabled() to scan the hugetlb_cma
>> array, could we just use a boolean as follows?  It can simply be set in
>> hugetlb_cma_reserve when we reserve CMA.
> 
> Maybe just use hugetlb_cma_size? If hugetlb_cma_size is not 0, someone is trying to use
> cma, then bootmem for gigantic pages will be totally ignored according to discussion here:
> https://lkml.org/lkml/2020/7/8/1288
> 
> if somebody sets a wrong hugetlb_cma_size which causes that cma is not reserved. 
> It is the fault of users? We just need to document hugetlb_cma will overwrite bootmem
> reservations?
> 

Yes, I think using hugetlb_cma_size would be sufficient.  If someone
specifies hugetlb_cma=<N> and hugepagesz=<gigantic_page_size> hugepages=<X>
that is wrong.  I don't think we need to worry about 'falling back' to
preallocating X gigantic pages if N is a bad value.  Or, even if the arch
does not support cma allocation.

I am working on a patch to check this earlier in command processing.  That
will make this check unnecessary.  However, that patch is based on new
command line processing code only in 5.8.  So, I think we still need to do
this so that it can be backported to stable.
-- 
Mike Kravetz