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

Vlastimil Babka
2018-11-05T16:55:28Z

On 11/5/18 5:19 PM, Konstantin Khlebnikov wrote:
>
>
> 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@yandex-team.ru>
>>
>> Makes sense regardless of warnings stuff.
>>
>> Acked-by: Vlastimil Babka <vbabka@suse.cz>
>>
>> But it must be moved below the GFP_KERNEL check!
>
> But kmalloc cannot handle it regardless of GFP.

Sure, but that's less problematic than skipping to vmalloc() for
!GFP_KERNEL. Especially for large sizes where it's likely that page
tables might get allocated (with GFP_KERNEL).

> 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;
> }

Probably should check also for __GFP_NOWARN.

> 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.

Dunno, what does Michal think?

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