linux-kernel - Re: [PATCH] lib/idr.c: Zero memory properly in idr_remove

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Date:	Tue, 13 Jan 2009 14:48:02 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	krh@...hat.com, stefanr@...6.in-berlin.de, dcm@....org,
	Nadia.Derbey@...l.net, linux1394-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, paulmck@...ibm.com
Subject: Re: [PATCH] lib/idr.c: Zero memory properly in idr_remove_all

On Mon, 12 Jan 2009 21:50:36 +0100
Manfred Spraul <manfred@...orfullife.com> wrote:

> Kristian H__gsberg wrote:
> > On Mon, 2009-01-12 at 20:53 +0100, Manfred Spraul wrote:
> >   
> >> Kristian H__gsberg wrote:
> >>     
> >>>   The problem
> >>> isn't about returning un-zeroed-out objects to the kmem cache, the
> >>> problem is returning them to the idr free list.
> >>>   
> >>>       
> >> I think this is wrong:
> >> The slab allocator assumes that the objects that are given to 
> >> kmem_cache_free() are properly constructed.
> >> I.e.: No additional constructor is called prior to returning the object 
> >> from the next kmem_cache_alloc() call.
> >>     
> >
> > That's fine, the ctor associated with the kmem cache is called, and in
> > the case of idr, it does a memset().
> >   
> No.
> As I said, the construtor is not called.
> An object that is given to kmem_cache_free() must be properly constructed.
> kmem_cache_free() just adds the obj pointer to a list, the next 
> kmem_cache_alloc returns the pointer.
> 
> This is also documented in mm/slab.c:
>  * The memory is organized in caches, one cache for each object type.
>  * (e.g. inode_cache, dentry_cache, buffer_head, vm_area_struct)
>  * Each cache consists out of many slabs (they are small (usually one
>  * page long) and always contiguous), and each slab contains multiple
>  * initialized objects.
>  *
>  * This means, that your constructor is used only for newly allocated
>  * slabs and you must pass objects with the same initializations to
>  * kmem_cache_free.
>  *
> 
> If the idr code passes uninitialized objects to kmem_cache_free(), then 
> the next kmem_cache_alloc will return a bad object.
> 

None of this got us much closer to fixing the bug ;)

What do we think of just removing the constructor and using
kmem_cache_zalloc()?

--- a/lib/idr.c~a
+++ a/lib/idr.c
@@ -121,7 +121,7 @@ int idr_pre_get(struct idr *idp, gfp_t g
 {
 	while (idp->id_free_cnt < IDR_FREE_MAX) {
 		struct idr_layer *new;
-		new = kmem_cache_alloc(idr_layer_cache, gfp_mask);
+		new = kmem_cache_zalloc(idr_layer_cache, gfp_mask);
 		if (new == NULL)
 			return (0);
 		move_to_free_list(idp, new);
@@ -623,16 +623,10 @@ void *idr_replace(struct idr *idp, void 
 }
 EXPORT_SYMBOL(idr_replace);
 
-static void idr_cache_ctor(void *idr_layer)
-{
-	memset(idr_layer, 0, sizeof(struct idr_layer));
-}
-
 void __init idr_init_cache(void)
 {
 	idr_layer_cache = kmem_cache_create("idr_layer_cache",
-				sizeof(struct idr_layer), 0, SLAB_PANIC,
-				idr_cache_ctor);
+				sizeof(struct idr_layer), 0, SLAB_PANIC, NULL);
 }
 
 /**
_

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