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

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Date:   Mon, 5 Nov 2018 19:19:28 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2] mm/kvmalloc: do not call kmalloc for size >
 KMALLOC_MAX_SIZE



On 05.11.2018 16:03, Vlastimil Babka wrote:
> On 11/1/18 11:09 AM, Konstantin Khlebnikov wrote:
>> Allocations over KMALLOC_MAX_SIZE could be served only by vmalloc.
>>
>> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
> 
> Makes sense regardless of warnings stuff.
> 
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> 
> But it must be moved below the GFP_KERNEL check!

But kmalloc cannot handle it regardless of GFP.

Ok maybe write something like this

if (size > KMALLOC_MAX_SIZE) {
	if (WARN_ON_ONCE((flags & GFP_KERNEL) != GFP_KERNEL)
		return NULL;
	goto do_vmalloc;
}

or fix that uncertainty right in vmalloc

For now comment in vmalloc declares

  *	Any use of gfp flags outside of GFP_KERNEL should be consulted
  *	with mm people.

=)

> 
>> ---
>>   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));
>>   }
>>
>