Re: [PATCH 0/3] [RFC] mm/vmalloc: fix possible exhaustion of vmalloc space

[Roman Peniaev]

On Mon, Mar 16, 2015 at 7:49 PM, Roman Peniaev <r.peniaev@xxxxxxxxx> wrote:
> On Mon, Mar 16, 2015 at 7:28 PM, Gioh Kim <gioh.kim@xxxxxxx> wrote:
>>
>>
>> 2015-03-13 ìí 9:12ì Roman Pen ì(ê) ì ê:
>>> Hello all.
>>>
>>> Recently I came across high fragmentation of vm_map_ram allocator: vmap_block
>>> has free space, but still new blocks continue to appear.  Further investigation
>>> showed that certain mapping/unmapping sequence can exhaust vmalloc space.  On
>>> small 32bit systems that's not a big problem, cause purging will be called soon
>>> on a first allocation failure (alloc_vmap_area), but on 64bit machines, e.g.
>>> x86_64 has 45 bits of vmalloc space, that can be a disaster.
>>
>> I think the problem you comments is already known so that I wrote comments about it as
>> "it could consume lots of address space through fragmentation".
>>
>> Could you tell me about your situation and reason why it should be avoided?
>
> In the first patch of this set I explicitly described the function,
> which exhausts
> vmalloc space without any chance to be purged: vm_map_ram allocator is
> greedy and firstly
> tries to occupy newly allocated block, even old blocks contain enough
> free space.
>
> This can be easily fixed if we put newly allocated block (which has
> enough space to
> complete further requests) to the tail of a free list, to give a
> chance to old blocks.
>
> Why it should be avoided?  Strange question.  For me it looks like a
> bug of an allocator,
> which should be fair and should not continuously allocate new blocks
> without lazy purging
> (seems vmap_lazy_nr and  __purge_vmap_area_lazy were created exactly
> for those reasons:
>  to avoid infinite allocations)


And if you are talking about your commit 364376383, which adds this comment

 * If you use this function for less than VMAP_MAX_ALLOC pages, it could be
 * faster than vmap so it's good.  But if you mix long-life and short-life
 * objects with vm_map_ram(), it could consume lots of address space through
 * fragmentation (especially on a 32bit machine).  You could see failures in
 * the end.  Please use this function for short-lived objects.

This is not that case, because if block is pinned, i.e. some pages are still
in use, we can't do anything with that.

I am talking about blocks, which are completely freed, but dirty.


--
Roman

>
>
> --
> Roman
>
>
>>
>>
>>>
>>> Fixing this I also did some tweaks in allocation logic of a new vmap block and
>>> replaced dirty bitmap with min/max dirty range values to make the logic simpler.
>>>
>>> I would like to receive comments on the following three patches.
>>>
>>> Thanks.
>>>
>>> Roman Pen (3):
>>>    mm/vmalloc: fix possible exhaustion of vmalloc space caused by
>>>      vm_map_ram allocator
>>>    mm/vmalloc: occupy newly allocated vmap block just after allocation
>>>    mm/vmalloc: get rid of dirty bitmap inside vmap_block structure
>>>
>>>   mm/vmalloc.c | 94 ++++++++++++++++++++++++++++++++++--------------------------
>>>   1 file changed, 54 insertions(+), 40 deletions(-)
>>>
>>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>>> Cc: Nick Piggin <npiggin@xxxxxxxxx>
>>> Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
>>> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
>>> Cc: David Rientjes <rientjes@xxxxxxxxxx>
>>> Cc: WANG Chao <chaowang@xxxxxxxxxx>
>>> Cc: Fabian Frederick <fabf@xxxxxxxxx>
>>> Cc: Christoph Lameter <cl@xxxxxxxxx>
>>> Cc: Gioh Kim <gioh.kim@xxxxxxx>
>>> Cc: Rob Jones <rob.jones@xxxxxxxxxxxxxxx>
>>> Cc: linux-mm@xxxxxxxxx
>>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>>> Cc: stable@xxxxxxxxxxxxxxx
>>>
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