Re: [PATCH][RFC] mm: warning message for vm_map_ram about vm size

[Zhang Yanfei <zhangyanfei@cn.fujitsu.com>]

On 03/10/2014 01:47 PM, Minchan Kim wrote:
> Hi Giho,
> 
> On Mon, Mar 10, 2014 at 01:57:07PM +0900, Gioh Kim wrote:
>> Hi,
>>
>> I have a failure of allocation of virtual memory on ARMv7 based platform.
>>
>> I called alloc_page()/vm_map_ram() for allocation/mapping pages.
>> Virtual memory space exhausting problem occurred.
>> I checked virtual memory space and found that there are too many 4MB chunks.
>>
>> I thought that if just one page in the 4MB chunk lives long, 
>> the entire chunk cannot be freed. Therefore new chunk is created again and again.
>>
>> In my opinion, the vm_map_ram() function should be used for temporary mapping
>> and/or short term memory mapping. Otherwise virtual memory is wasted.
>>
>> I am not sure if my opinion is correct. If it is, please add some warning message
>> about the vm_map_ram().
>>
>>
>>
>> ---8<---
>>
>> Subject: [PATCH] mm: warning comment for vm_map_ram
>>
>> vm_map_ram can occur locking of virtual memory space
>> because if only one page lives long in one vmap_block,
>> it takes 4MB (1024-times more than one page) space.
> 
> For clarification, vm_map_ram has fragment problem because it
> couldn't purge a chunk(ie, 4M address space) if there is a pinning
> object in that addresss space so it could consume all VMALLOC
> address space easily.
> 
> We can fix the fragementaion problem with using vmap instead of
> vm_map_ram but it wouldn't a good solution because vmap is much
> slower than vm_map_ram for VMAP_MAX_ALLOC below. In my x86 machine,
> vm_map_ram is 5 times faster than vmap.
> 
> AFAICR, some proprietary GPU driver uses that function heavily so
> performance would be really important so I want to stick to use
> vm_map_ram.
> 
> Another option is that caller should separate long-life and short-life
> object and use vmap for long-life but vm_map_ram for short-life.
> But it's not a good solution because it's hard for allocator layer
> to detect it that how customer lives with the object.

Indeed. So at least the note comment should be added.

> 
> So I thought to fix that problem with revert [1] and adding more
> logic to solve fragmentation problem and make bitmap search
> operation more efficient by caching the hole. It might handle
> fragmentation at the moment but it would make more IPI storm for
> TLB flushing as time goes by so that it would mitigate API itself
> so using for only temporal object is too limited but it's best at the
> moment. I am supporting your opinion.
> 
> Let's add some notice message to user.
> 
> [1] [3fcd76e8028, mm/vmalloc.c: remove dead code in vb_alloc]
> 
>>
>> Change-Id: I6f5919848cf03788b5846b7d850d66e4d93ac39a
>> Signed-off-by: Gioh Kim <gioh.kim@lge.com>
>> ---
>>  mm/vmalloc.c |    4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> index 0fdf968..2de1d1b 100644
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -1083,6 +1083,10 @@ EXPORT_SYMBOL(vm_unmap_ram);
>>   * @node: prefer to allocate data structures on this node
>>   * @prot: memory protection to use. PAGE_KERNEL for regular RAM
>>   *
>> + * This function should be used for TEMPORARY mapping. If just one page lives i
>> + * long, it would occupy 4MB vm size permamently. 100 pages (just 400KB) could
>> + * takes 400MB with bad luck.
>> + *
> 
>     If you use this function for below VMAP_MAX_ALLOC pages, it could be faster
>     than vmap so it's good but if you mix long-life and short-life object
>     with vm_map_ram, it could consume lots of address space by fragmentation(
>     expecially, 32bit machine) so you could see failure in the end.
>     So, please use this function for short-life object.

Minchan's is better. So I suggest Giho post another patch with this comment
and take what Minchan said above to the commit log. And you can feel free to
add:

Reviewed-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>

Thanks.

> 
>>   * Returns: a pointer to the address that has been mapped, or %NULL on failure
>>   */
>>  void *vm_map_ram(struct page **pages, unsigned int count, int node, pgprot_t prot)
>> --
>> 1.7.9.5
>>
>> Gioh Kim / 김 기 오
>> Research Engineer
>> Advanced OS Technology Team
>> Software Platform R&D Lab.
>> Mobile: 82-10-7322-5548  
>> E-mail: gioh.kim@lge.com 
>> 19, Yangjae-daero 11gil
>> Seocho-gu, Seoul 137-130, Korea
>>
>>
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> 


-- 
Thanks.
Zhang Yanfei

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