linux-kernel - Re: [PATCH 2] mm/kvmalloc: do not call kmalloc for size > KMALLOC_MAX

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Date:   Thu, 1 Nov 2018 13:48:17 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2] mm/kvmalloc: do not call kmalloc for size >
 KMALLOC_MAX_SIZE



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@...dex-team.ru>
> 
> Acked-by: Michal Hocko <mhocko@...e.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));
>>   }
>>
>