linux-kernel - Re: [PATCH RFC] mm: vmalloc: do not allow kzalloc to fail

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Date:   Sat, 22 Dec 2018 09:04:21 +0100
From:   Nicholas Mc Guire <der.herr@...r.at>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Nicholas Mc Guire <hofrat@...dl.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Chintan Pandya <cpandya@...eaurora.org>,
        Michal Hocko <mhocko@...e.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Arun KS <arunks@...eaurora.org>, Joe Perches <joe@...ches.com>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] mm: vmalloc: do not allow kzalloc to fail

On Fri, Dec 21, 2018 at 01:58:39PM -0800, David Rientjes wrote:
> On Thu, 20 Dec 2018, Nicholas Mc Guire wrote:
> 
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 871e41c..1c118d7 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -1258,7 +1258,7 @@ void __init vmalloc_init(void)
> >  
> >  	/* Import existing vmlist entries. */
> >  	for (tmp = vmlist; tmp; tmp = tmp->next) {
> > -		va = kzalloc(sizeof(struct vmap_area), GFP_NOWAIT);
> > +		va = kzalloc(sizeof(*va), GFP_NOWAIT | __GFP_NOFAIL);
> >  		va->flags = VM_VM_AREA;
> >  		va->va_start = (unsigned long)tmp->addr;
> >  		va->va_end = va->va_start + tmp->size;
> 
> Hi Nicholas,
> 
> You're right that this looks wrong because there's no guarantee that va is 
> actually non-NULL.  __GFP_NOFAIL won't help in init, unfortunately, since 
> we're not giving the page allocator a chance to reclaim so this would 
> likely just end up looping forever instead of crashing with a NULL pointer 
> dereference, which would actually be the better result.
>
tried tracing the __GFP_NOFAIL path and had concluded that it would
end in out_of_memory() -> panic("System is deadlocked on memory\n");
which also should point cleanly to the cause - but I´m actually not
that sure if that trace was correct in all cases.
 
> You could do
> 
> 	BUG_ON(!va);
> 
> to make it obvious why we crashed, however.  It makes it obvious that the 
> crash is intentional rather than some error in the kernel code.

makes sense - that atleast makes it imediately clear from the code
that there is no way out from here.

thx!
hofrat