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

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Message-ID: <alpine.DEB.2.21.1812211356040.219499@chino.kir.corp.google.com>
Date:   Fri, 21 Dec 2018 13:58:39 -0800 (PST)
From:   David Rientjes <rientjes@...gle.com>
To:     Nicholas Mc Guire <hofrat@...dl.org>
cc:     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 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.

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.