linux-kernel - Re: [PATCH] [v2] nvdimm-btt: fix several memleaks

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Message-ID: <62f848c1.48243.18c86366e76.Coremail.dinghao.liu@zju.edu.cn>
Date: Wed, 20 Dec 2023 15:52:43 +0800 (GMT+08:00)
From: dinghao.liu@....edu.cn
To: "Ira Weiny" <ira.weiny@...el.com>
Cc: "Dan Williams" <dan.j.williams@...el.com>, 
	"Vishal Verma" <vishal.l.verma@...el.com>, 
	"Dave Jiang" <dave.jiang@...el.com>, nvdimm@...ts.linux.dev, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [v2] nvdimm-btt: fix several memleaks

> dinghao.liu@ wrote:
> > > Ira Weiny wrote:
> > > > Dinghao Liu wrote:

[snip]

> > > > 
> > > > This does not quite work.
> > > > 
> > > > free_arenas() is used in the error paths of create_arenas() and
> > > > discover_arenas().  In those cases devm_kfree() is probably a better way
> > > > to clean up this.
> > 
> > Here I'm a little confused about when devm_kfree() should be used.
> 
> Over all it should be used whenever memory is allocated for the lifetime
> of the device.
> 
> > Code in btt_init() implies that resources allocated by devm_* could be
> > auto freed in both error and success paths of probe/attach (e.g., btt 
> > allocated by devm_kzalloc is never freed by devm_kfree).
> > Using devm_kfree() in free_arenas() is ok for me, but I want to know
> > whether not using devm_kfree() constitutes a bug.
> 
> Unfortunately I'm not familiar enough with this code to know for sure.
> 
> After my quick checks before I thought it was.  But each time I look at it
> I get confused.  This is why I was thinking maybe not using devm_*() and
> using no_free_ptr() may be a better solution to make sure things don't
> leak without changing the success path (which is likely working fine
> because no bugs have been found.)

We have the same confusion here... I find a discussion about this problem,
which implies that not using devm_kfree() may delay the release, but the memory will be freed later and no memory is leaked:

https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg2009561.html

> > 
> > > We might want to look at using no_free_ptr() in this code.  See this
> > > patch[1] for an example of how to inhibit the cleanup and pass the
> > > pointer on when the function succeeds.
> > > 
> > > [1]
> > > https://lore.kernel.org/all/170261791914.1714654.6447680285357545638.stgit@dwillia2-xfh.jf.intel.com/
> > > 
> > > Ira
> > 
> > Thanks for this example. But it seems that no_free_ptr() is used to
> > handle the scope based resource management. Changes in this patch does
> > not introduce this feature. Do I understand this correctly?
> 
> You do understand but I was thinking that perhaps using no_free_ptr()
> rather than devm_*() might be an easier way to fix this bug without trying
> to decode the lifetime of everything.
> 

My concern is that no_free_ptr() may not be able to completely fix all
memleaks because some of them are triggered in (part of) success paths (e.g.,
when btt_freelist_init succeeds but btt_rtt_init fails, discover_arenas still needs to clean up the memory allocated in btt_freelist_init).

I checked the design of no_free_ptr(), and it seems that it will generate
a new pointer on success and the memory still leaks in the above case. 
Therefore, I think using devm_*() is still the best solution for this bug. 

Regards,
Dinghao