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

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Message-ID: <alpine.DEB.2.10.1503131624060.7827@chino.kir.corp.google.com>
Date:	Fri, 13 Mar 2015 16:24:32 -0700 (PDT)
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, David Rientjes 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.
> 

Jens, are these two patches going to be merged into linux-block?

Thanks.
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