Re: [PATCH] mm: drop VM_BUG_ON from __get_free_pages

[Vlastimil Babka <vbabka@suse.cz>]

On 06/27/2018 09:54 AM, Michal Hocko wrote:
> On Wed 27-06-18 09:50:01, Vlastimil Babka wrote:
>> On 06/27/2018 09:34 AM, Michal Hocko wrote:
>>> On Tue 26-06-18 10:04:16, Andrew Morton wrote:
>>>
>>> And as I've argued before the code would be wrong regardless. We would
>>> leak the memory or worse touch somebody's else kmap without knowing
>>> that.  So we have a choice between a mem leak, data corruption k or a
>>> silent fixup. I would prefer the last option. And blowing up on a BUG
>>> is not much better on something that is easily fixable. I am not really
>>> convinced that & ~__GFP_HIGHMEM is something to lose sleep over.
>>
>> Maybe put the fixup into a "#ifdef CONFIG_HIGHMEM" block and then modern
>> systems won't care? In that case it could even be if (WARN_ON_ONCE(...))
>> so future cases with wrong expectations would become known.
> 
> Yes that could be done as well. Or maybe we can make __GFP_HIGHMEM 0 for
> !HIGHMEM systems. Does something really rely on it being non-zero?

I guess gfp_zone() would have to be checked, dunno about the rewrite of
GFP_ZONE_TABLE (CCing people).
In general checks like "if (flags & __GFP_HIGHMEM)" would become false,
which probably should not be a problem, unless something expect the flag
to be there and errors out if it isn't.