Re: [Drbd-dev] [patch 1/2] block, drbd: fix drbd_req_new() initialization

[Jens Axboe <axboe@fb.com>]

On 03/07/2015 06:27 PM, David Rientjes wrote:
> On Sat, 7 Mar 2015, Jens Axboe wrote:
> 
>>>>> mempool_alloc() does not support __GFP_ZERO since elements may come from
>>>>> memory that has already been released by mempool_free().
>>>>>
>>>>> Remove __GFP_ZERO from mempool_alloc() in drbd_req_new() and properly
>>>>> initialize it to 0.
>>>>
>>>> You should add it to mempool instead, avoid having this issue show up for
>>>> other folks as well. It'd be trivial to do. Normal ->alloc() should honor
>>>> __GFP_ZERO, just do the same manually for removing an item from the internal
>>>> pool.
>>>>
>>>
>>> Umm, it's not trivial to do and wouldn't make sense to do it.  Mempools 
>>
>> Uhm, it would make sense, though.
>>
> 
> Disagree, I don't think we should extend mempool to know the element size, 
> modify every user of mempool to pass it in, and keep it consistent with 
> mempool_alloc_t for the benefit of __GFP_ZERO for this one buggy caller.  
> Most users don't need __GFP_ZERO and just overwrite the entire element 
> after mempool_alloc() and it would be an unnecessary overhead to even 
> check for the bit set.  So it wouldn't make sense in terms of performance 
> or maintainability.

My point is that conceptually, of course it makes sense to do and it
_should_ do it. We don't have the size, too bad, I don't disagree that
adding it just for this is necessarily the best idea.

>>> don't know the element size, in other words it wouldn't know the length to 
>>> memset() to 0 for mempool_alloc().  It shouldn't be modified to know the 
>>> element size since elements are allocated by the implementation of 
>>> mempool_alloc_t and they could easily become inconsistent.  This patch is 
>>> what you want to merge, really.
>>>
>>
>> I forgot we don't have the size in there. Then I would suggest adding a
>> WARN_ON() for __GFP_ZERO being set in mempool_alloc(), at the very least.
>>
> 
> There is, it's a VM_WARN_ON_ONCE() that will show up if you configure 
> CONFIG_DEBUG_VM.

OK, that's good enough then.

-- 
Jens Axboe