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

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Message-ID: <13ffb3fd.41cd7.18c7c3b52bf.Coremail.dinghao.liu@zju.edu.cn>
Date: Mon, 18 Dec 2023 17:21:51 +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

> Ira Weiny wrote:
> > Dinghao Liu wrote:
> 
> [snip]
> 
> -static int btt_freelist_init(struct arena_info *arena)
> +static int btt_freelist_init(struct device *dev, struct arena_info *arena)
> 
> Both struct arena_info and struct btt contain references to struct nd_btt
> which is the device you are passing down this call stack.
> 
> Just use the device in the arena/btt rather than passing a device
> structure.  That makes the code easier to read and ensures that the device
> associated with this arena or btt is used.

Thanks for this suggestion! I will fix this in the v3 patch.

> [snip]
> > >  
> > > -static int btt_maplocks_init(struct arena_info *arena)
> > > +static int btt_maplocks_init(struct device *dev, struct arena_info *arena)
> > >  {
> > >  	u32 i;
> > >  
> > > -	arena->map_locks = kcalloc(arena->nfree, sizeof(struct aligned_lock),
> > > +	arena->map_locks = devm_kcalloc(dev, arena->nfree, sizeof(struct aligned_lock),
> > >  				GFP_KERNEL);
> > >  	if (!arena->map_locks)
> > >  		return -ENOMEM;
> > > @@ -805,9 +805,6 @@ static void free_arenas(struct btt *btt)
> > >  
> > >  	list_for_each_entry_safe(arena, next, &btt->arena_list, list) {
> > >  		list_del(&arena->list);
> > > -		kfree(arena->rtt);
> > > -		kfree(arena->map_locks);
> > > -		kfree(arena->freelist);
> > 
> > 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.
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.

> > 
> > However...
> > 
> > >  		debugfs_remove_recursive(arena->debugfs_dir);
> > >  		kfree(arena);
> > 
> > Why can't arena be allocated with devm_*?
> > 
> > We need to change this up a bit more to handle the error path vs regular
> > device shutdown free (automatic devm frees).
> 

At first, I think the use of arena is correct. Therefore, allocating arena
with devm_* should be a code style optimization. However, I rechecked discover_arenas and found arena might also be leaked (e.g., if alloc_arena() fails in the second loop, the previously allocated resources in the loop is leaked). The correct code could be found in create_arenas(), where free_arenas is called on failure of alloc_arena().

To fix this issue, I think it's better to change the 'goto out_super' tag
to 'goto out'. We could also use devm_* for arena to simplify the error path
in discover_arenas(). 

> 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?

Regards,
Dinghao