linux-kernel - Re: [patch 1/2] block, drbd: fix drbd_req

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Message-ID: <alpine.DEB.2.10.1503071724050.4711@chino.kir.corp.google.com>
Date:	Sat, 7 Mar 2015 17:27:43 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Jens Axboe <axboe@...com>
cc:	Lars Ellenberg <drbd-dev@...ts.linbit.com>,
	linux-kernel@...r.kernel.org, drbd-user@...ts.linbit.com
Subject: Re: [patch 1/2] block, drbd: fix drbd_req_new() initialization

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.

> > 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.
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