[dm-devel] [PATCH] SLUB: Do not fallback to mininum order if __GFP_NORETRY is set

[Vlastimil Babka]

On 04/20/2018 04:53 PM, Christopher Lameter wrote:
> On Thu, 19 Apr 2018, Michal Hocko wrote:
> 
>> Overriding __GFP_NORETRY is just a bad idea. It will make the semantic
>> of the flag just more confusing. Note there are users who use
>> __GFP_NORETRY as a way to suppress heavy memory pressure and/or the OOM
>> killer. You do not want to change the semantic for them.
> 
> Redoing the allocation after failing a large order alloc is a retry. I
> would say its confusing right now because a retry occurs despite
> specifying GFP_NORETRY,
> 
>> Besides that the changelog is less than optimal. What is the actual
>> problem? Why somebody doesn't want a fallback? Is there a configuration
>> that could prevent the same?
> 
> The problem is that SLUB does not honor GFP_NORETRY. The semantics of
> GFP_NORETRY are not followed.

The caller might want SLUB to try hard to get that high-order page that
will minimize memory waste (e.g. 2MB page for 3 640k objects), and
__GFP_NORETRY will kill the effort on allocating that high-order page.

Thus, using __GPF_NORETRY for "please give me a space-optimized object,
or nothing (because I have a fallback that's better than wasting memory,
e.g. by using 1MB page for 640kb object)" is not ideal.

Maybe __GFP_RETRY_MAYFAIL is a better fit? Or perhaps indicate this
situation to SLUB with e.g. __GFP_COMP, although that's rather ugly?