Re: [PATCH 2] mm/kvmalloc: do not call kmalloc for size > KMALLOC_MAX_SIZE

[Konstantin Khlebnikov]

On 01.11.2018 13:24, Michal Hocko wrote:
> On Thu 01-11-18 13:09:16, Konstantin Khlebnikov wrote:
> > Allocations over KMALLOC_MAX_SIZE could be served only by vmalloc.
>
> I would go on and say that allocations with sizes too large can actually
> trigger a warning (once you have posted in the previous version outside
> of the changelog area) because that might be interesting to people -
> there are deployments to panic on warning and then a warning is much
> more important.

It seems that warning isn't completely valid.


__alloc_pages_slowpath() handles this more gracefully:

	/*
	 * In the slowpath, we sanity check order to avoid ever trying to
	 * reclaim >= MAX_ORDER areas which will never succeed. Callers may
	 * be using allocators in order of preference for an area that is
	 * too large.
	 */
	if (order >= MAX_ORDER) {
		WARN_ON_ONCE(!(gfp_mask & __GFP_NOWARN));
		return NULL;
	}


Fast path is ready for order >= MAX_ORDER


Problem is in node_reclaim() which is called earlier than __alloc_pages_slowpath()
from surprising place - get_page_from_freelist()


Probably node_reclaim() simply needs something like this:

	if (order >= MAX_ORDER)
		return NODE_RECLAIM_NOSCAN;


> > Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
>
> Thanks!
>
> > ---
> >   mm/util.c |    4 ++++
> >   1 file changed, 4 insertions(+)
> >
> > diff --git a/mm/util.c b/mm/util.c
> > index 8bf08b5b5760..f5f04fa22814 100644
> > --- a/mm/util.c
> > +++ b/mm/util.c
> > @@ -392,6 +392,9 @@ void *kvmalloc_node(size_t size, gfp_t flags, int node)
> >   	gfp_t kmalloc_flags = flags;
> >   	void *ret;
> >   
> > +	if (size > KMALLOC_MAX_SIZE)
> > +		goto fallback;
> > +
> >   	/*
> >   	 * vmalloc uses GFP_KERNEL for some internal allocations (e.g page tables)
> >   	 * so the given set of flags has to be compatible.
> > @@ -422,6 +425,7 @@ void *kvmalloc_node(size_t size, gfp_t flags, int node)
> >   	if (ret || size <= PAGE_SIZE)
> >   		return ret;
> >   
> > +fallback:
> >   	return __vmalloc_node_flags_caller(size, node, flags,
> >   			__builtin_return_address(0));
> >   }