linux-kernel - Re: [PATCH 1/1] mm/vmalloc.c: Add an error message if two areas overlap

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Date:   Fri, 10 Jul 2020 21:01:38 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Hillf Danton <hdanton@...a.com>,
        Michal Hocko <mhocko@...e.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 1/1] mm/vmalloc.c: Add an error message if two areas
 overlap

On Fri, Jul 10, 2020 at 10:00:30PM +0200, Uladzislau Rezki wrote:
> On Fri, Jul 10, 2020 at 08:44:06PM +0100, Matthew Wilcox wrote:
> > On Fri, Jul 10, 2020 at 09:40:42PM +0200, Uladzislau Rezki (Sony) wrote:
> > > Before triggering a BUG() it would be useful to understand
> > > how two areas overlap between each other. Print information
> > > about start/end addresses of both VAs and their addresses.
> > > 
> > > For example if both are identical it could mean double free.
> > > 
> > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> > > ---
> > >  mm/vmalloc.c | 10 +++++++++-
> > >  1 file changed, 9 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > index 5a2b55c8dd9a..1679b01febcd 100644
> > > --- a/mm/vmalloc.c
> > > +++ b/mm/vmalloc.c
> > > @@ -549,8 +549,13 @@ find_va_links(struct vmap_area *va,
> > >  		else if (va->va_end > tmp_va->va_start &&
> > >  				va->va_start >= tmp_va->va_end)
> > >  			link = &(*link)->rb_right;
> > > -		else
> > > +		else {
> > > +			pr_err("Overlaps: 0x%px(0x%lx-0x%lx), 0x%px(0x%lx-0x%lx)\n",
> > > +				va, va->va_start, va->va_end, tmp_va,
> > > +				tmp_va->va_start, tmp_va->va_end);
> > 
> > It might be helpful to have a "vmalloc:" prefix to that string to indicate
> > where to start searching.  And I don't think we're supposed to use %px
> > without a really good justification these days.
> >
> That makes sense, i will add such prefix for sure. As for %px i can just
> use a casting to (unsigned long) and 0x%lx prefix.

That's not the point.  Linus wants us to not display actual pointers to
the user.