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

Konstantin Khlebnikov
2018-11-01T10:48:20Z

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@yandex-team.ru>
>
> Acked-by: Michal Hocko <mhocko@suse.com>
>
> 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));
>> }
>>
>