linux-kernel - Re: DEFINE_IDA causing memory leaks? (was Re: [PATCH 1/2] virtio: fix memory leak of virtio ida cache layers)

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Date:	Thu, 17 Sep 2015 10:58:29 -0700
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Tejun Heo <htejun@...il.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>, Suman Anna <s-anna@...com>,
	Ohad Ben-Cohen <ohad@...ery.com>, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	linux-scsi@...r.kernel.org, open-iscsi@...glegroups.com
Subject: Re: DEFINE_IDA causing memory leaks? (was  Re: [PATCH 1/2] virtio:
 fix memory leak of virtio ida cache layers)

On Thu, 2015-09-17 at 13:15 -0400, Tejun Heo wrote:
> Hello,
> 
> On Thu, Sep 17, 2015 at 09:48:37AM -0700, James Bottomley wrote:
> > Well, there's an easy fix for that.  We could have ida_remove() actually
> > free the bitmap and not cache it if it's the last layer.  That way ida
> > would naturally empty and we wouldn't need a destructor.   Tejun, would
> > that work?
> 
> Yeah, that definitely is one way to go about it.  It kinda muddles the
> purpose of ida_destroy() tho.  I suppose we can rename it to
> idr_remove_all() and then do the same to idr.  I'm not particularly
> objecting to all that but what's wrong with just calling idr_destroy()
> on exit paths?  If missing the call in modules is an issue, maybe we
> can just annotate idr/ida with debugobj?

The argument is that we shouldn't have to explicitly destroy a
statically initialized object, so 

DEFINE_IDA(someida);

Should just work without having to explicitly do

ida_destory(someida);

somewhere in the exit code.  It's about usage patterns.  Michael's
argument is that if we can't follow the no destructor pattern for
DEFINE_IDA() then we shouldn't have it at all, because it's confusing
kernel design patterns.  The pattern we would have would be

struct ida someida:

ida_init(&someida);

...

ida_destroy(&someida);

so the object explicitly has a constructor matched to a destructor.

James


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