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    SubjectRe: [PATCH v3 4/4] mm/vmap: move BUG_ON() check to the unlink_va()
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    On Wed, May 29, 2019 at 03:58:17PM +0200, Uladzislau Rezki wrote:
    > Hello, Roman!
    >
    > > > Move the BUG_ON()/RB_EMPTY_NODE() check under unlink_va()
    > > > function, it means if an empty node gets freed it is a BUG
    > > > thus is considered as faulty behaviour.
    > >
    > > It's not exactly clear from the description, why it's better.
    > >
    > It is rather about if "unlink" happens on unhandled node it is
    > faulty behavior. Something that clearly written in stone. We used
    > to call "unlink" on detached node during merge, but after:
    >
    > [PATCH v3 3/4] mm/vmap: get rid of one single unlink_va() when merge
    >
    > it is not supposed to be ever happened across the logic.
    >
    > >
    > > Also, do we really need a BUG_ON() in either place?
    > >
    > Historically we used to have the BUG_ON there. We can get rid of it
    > for sure. But in this case, it would be harder to find a head or tail
    > of it when the crash occurs, soon or later.
    >
    > > Isn't something like this better?
    > >
    > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
    > > index c42872ed82ac..2df0e86d6aff 100644
    > > --- a/mm/vmalloc.c
    > > +++ b/mm/vmalloc.c
    > > @@ -1118,7 +1118,8 @@ EXPORT_SYMBOL_GPL(unregister_vmap_purge_notifier);
    > >
    > > static void __free_vmap_area(struct vmap_area *va)
    > > {
    > > - BUG_ON(RB_EMPTY_NODE(&va->rb_node));
    > > + if (WARN_ON_ONCE(RB_EMPTY_NODE(&va->rb_node)))
    > > + return;
    > >
    > I was thinking about WARN_ON_ONCE. The concern was about if the
    > message gets lost due to kernel ring buffer. Therefore i used that.
    > I am not sure if we have something like WARN_ONE_RATELIMIT that
    > would be the best i think. At least it would indicate if a warning
    > happens periodically or not.
    >
    > Any thoughts?

    Hello, Uladzislau!

    I don't have a strong opinion here. If you're worried about losing the message,
    WARN_ON() should be fine here. I don't think that this event will happen often,
    if at all.

    Thanks!

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