On Mon, 2012-06-25 at 15:10 +0400, Glauber Costa wrote:
> On 06/25/2012 01:53 PM, Li Zhong wrote:
> > SLUB duplicates the cache name in kmem_cache_create(). However if the
> > cache could be merged to others during early booting, the name pointer
> > is saved in saved_alias list, and the string needs to be kept valid
> > before slab_sysfs_init() is called.
> >
> > This patch tries to duplicate the cache name in saved_alias list, so
> > that the cache name could be safely kfreed after calling
> > kmem_cache_create(), if that name is kmalloced.
> >
> > Signed-off-by: Li Zhong <zhong@xxxxxxxxxxxxxxxxxx>
> > ---
> > mm/slub.c | 6 ++++++
> > 1 files changed, 6 insertions(+), 0 deletions(-)
> >
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 8c691fa..3dc8ed5 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -5373,6 +5373,11 @@ static int sysfs_slab_alias(struct kmem_cache *s,
> > const char *name)
> >
> > al->s = s;
> > al->name = name;
> > + al->name = kstrdup(name, GFP_KERNEL);
> > + if (!al->name) {
> > + kfree(al);
> > + return -ENOMEM;
> > + }
> > al->next = alias_list;
> > alias_list = al;
> > return 0;
> > @@ -5409,6 +5414,7 @@ static int __init slab_sysfs_init(void)
> > if (err)
> > printk(KERN_ERR "SLUB: Unable to add boot slab alias"
> > " %s to sysfs\n", s->name);
> > + kfree(al->name);
> > kfree(al);
> > }
> >
> >
>
> What's unsafe about the current state of affairs ?
> Whenever we alias, we'll increase the reference counter.
> kmem_cache_destroy will only actually destroy the structure whenever
> that refcnt reaches zero.
>
> This means that kfree shouldn't happen until then. So what is exactly
> that you are seeing?
Maybe I didn't describe it clearly ... It is only about the name string
passed into kmem_cache_create() during early boot.
kmem_cache_create() checks whether it is mergeable before creating one.
If not mergeable, the name is duplicated: n = kstrdup(name, GFP_KERNEL);
If it is mergeable, it calls sysfs_slab_alias(). If the sysfs is ready
(slab_state == SYSFS ), then the name is duplicated (or dropped if no
SYSFS support ) in sysfs_create_link() for use.
For the above cases, we could safely kfree the name string after calling
cache create.
However, During early boot, before sysfs is ready ( slab_state <
SYSFS ), the sysfs_slab_alias() saves the pointer of name in the
alias_list. And those entries in the list are added to sysfs later after
slab_sysfs_init() is called. So we need to keep the name string valid
until slab_sysfs_init() is called to set up the sysfs stuff. By
duplicating the name string here also, we are able to kfree the name
string after calling the cache create.
>
> Now, if you ask me, keeping the name around in user-visible files like
> /proc/slabinfo for caches that are removed already can be a bit
> confusing (that is because we don't add aliases to the slab_cache list)
>
> If you want to touch this, one thing you can do is to keep a list of
> names bundled in an alias. If an alias is removed, you free that name.
> If that name is the representative name of the bundle, you move to the
> next one.
>
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>