Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers

[John Hubbard]

On 01/19/2017 12:45 AM, Michal Hocko wrote:
> On Thu 19-01-17 00:37:08, John Hubbard wrote:
> > On 01/18/2017 12:21 AM, Michal Hocko wrote:
> > > On Tue 17-01-17 21:59:13, John Hubbard wrote:
> [...]
> > > >  * Reclaim modifiers - __GFP_NORETRY and __GFP_NOFAIL should not be passed in.
> > > >  * Passing in __GFP_REPEAT is supported, but note that it is ignored for small
> > > >  * (<=64KB) allocations, during the kmalloc attempt.
> > >
> > > > __GFP_REPEAT is fully
> > > >  * honored for  all allocation sizes during the second part: the vmalloc attempt.
> > >
> > > this is not true to be really precise because vmalloc doesn't respect
> > > the given gfp mask all the way down (look at the pte initialization).
> >
> > I'm having some difficulty in locating that pte initialization part, am I on
> > the wrong code path? Here's what I checked, before making the claim about
> > __GFP_REPEAT being honored:
> >
> > kvmalloc_node
> >   __vmalloc_node_flags
> >     __vmalloc_node
> >       __vmalloc_node_range
> >         __vmalloc_area_node
> 	    map_vm_area
> 	      vmap_page_range
> 	        vmap_page_range_noflush
> 		  vmap_pud_range
> 		    pud_alloc
> 		      __pud_alloc
> 		        pud_alloc_one
>
> pud will be allocated but the same pattern repeats on the pmd and pte
> levels. This is btw. one of the reasons why vmalloc with gfp flags is
> tricky!

Yes, I see that now, thank you for explaining, much appreciated. The flags are left 
way behind in the code path.

So that leaves us with maybe this for documentation?

 * Reclaim modifiers - __GFP_NORETRY and __GFP_NOFAIL should not be passed in.
 * Passing in __GFP_REPEAT is supported, and will cause the following behavior:
 * for larger (>64KB) allocations, the first part (kmalloc) will do some
 * retrying, before falling back to vmalloc.


> moreover
> >             alloc_pages_node
>
> this is order-0 request so...
>
> >               __alloc_pages_node
> >                 __alloc_pages
> >                   __alloc_pages_nodemask
> >                     __alloc_pages_slowpath
> >
> >
> > ...and __alloc_pages_slowpath does the __GFP_REPEAT handling:
> >
> >     /*
> >      * Do not retry costly high order allocations unless they are
> >      * __GFP_REPEAT
> >      */
> >     if (order > PAGE_ALLOC_COSTLY_ORDER && !(gfp_mask & __GFP_REPEAT))
> >         goto nopage;
>
> ... this doesn't apply

yes, true.

thanks
john h

>