Re: [PATCH 5/8] mm, page_alloc: make THP-specific decisions more generic

[Vlastimil Babka]

On 07/20/2016 01:10 AM, David Rientjes wrote:
> On Mon, 18 Jul 2016, Vlastimil Babka wrote:
>
> > This means we can further distinguish allocations that are costly order *and*
> > additionally include the __GFP_NORETRY flag. As it happens, GFP_TRANSHUGE
> > allocations do already fall into this category. This will also allow other
> > costly allocations with similar high-order benefit vs latency considerations to
> > use this semantic. Furthermore, we can distinguish THP allocations that should
> > try a bit harder (such as from khugepageed) by removing __GFP_NORETRY, as will
> > be done in the next patch.
> >
> > Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> > Acked-by: Michal Hocko <mhocko@xxxxxxxx>
>
> I think this is fine, but I would hope that we could check
> gfp_pfmemalloc_allowed() before compacting and failing even for costly
> orders when otherwise the first get_page_from_freelist() in the slowpath
> may have succeeded due to watermarks.

Hm ok, I will add it for the sake of avoiding goto nopage where 
previously it would have tried alloc without watermarks, as that would 
be unintended side-effect of the series... although I have some doubts 
about sanity of such scenarios (wants a costly order, can 
reclaim/compact but only with __GFP_NORETRY, yet is allowed to avoid 
watermarks?). Do you know about examples of such callers and think they 
do the right thing?

Thanks,
Vlastimil