Re: [RFC v2 1/3] mm/vmalloc: fix possible exhaustion of vmalloc space caused by vm_map_ram allocator

[Roman Peniaev <r.peniaev@gmail.com>]

On Wed, Mar 25, 2015 at 7:00 AM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> On Thu, 19 Mar 2015 23:04:39 +0900 Roman Pen <r.peniaev@gmail.com> wrote:
>
>> If suitable block can't be found, new block is allocated and put into a head
>> of a free list, so on next iteration this new block will be found first.
>>
>> ...
>>
>> Cc: stable@vger.kernel.org
>>
>> ...
>>
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -837,7 +837,7 @@ static struct vmap_block *new_vmap_block(gfp_t gfp_mask)
>>
>>       vbq = &get_cpu_var(vmap_block_queue);
>>       spin_lock(&vbq->lock);
>> -     list_add_rcu(&vb->free_list, &vbq->free);
>> +     list_add_tail_rcu(&vb->free_list, &vbq->free);
>>       spin_unlock(&vbq->lock);
>>       put_cpu_var(vmap_block_queue);
>>
>
> I'm not sure about the cc:stable here.  There is potential for
> unexpected side-effects

Only one potential side-effect I see is that allocator has to iterate
up to 6 (7 on 64-bit systems) blocks in a free list two times.
The second patch fixes this by occupying the block right away after
allocation.  But even the second patch is not applied - iterating 6 (7)
blocks (and this is the worst and rare case) is not a big deal comparing
to the size of a free list, which increases over time, if this patch was
not applied.

I can compare the behaviour of the allocator, which puts new blocks to the
head of a free list, with the tetris game: sooner or later coming blocks
will reach the top, and you will lose, even if you are the champion.

> and I don't *think* people are hurting from
> this issue in real life.  Or maybe I'm wrong about that?

Yes, probably they are not.  I showed one special synthetic scenario, which
works pretty well and exhausts the virtual space very fast, another scenario
is a random one, which also works, but very slow.

I think drivers tend only to preallocate (not frequent usage) or to pass
sequential sizes to vm_map_ram.  In these cases everything will be fine.
Also free list is a CPU variable.  Good and fast reproduction happens only
if you bind a vm_map_ram call to the CPU or use uniprocessor system.

Probably the conjunction of all of these reasons hid the problem for a
long time.  But I tend to think that this is a bug, long-standing bug.

--
Roman

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