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Re: [PATCH v2] mm: module_alloc: check if size is 0

To: Ralf Baechle <ralf@linux-mips.org>
Subject: Re: [PATCH v2] mm: module_alloc: check if size is 0
From: Andrew Morton <akpm@linux-foundation.org>
Date: Thu, 27 Jun 2013 15:23:35 -0700
Cc: Veli-Pekka Peltola <veli-pekka.peltola@bluegiga.com>, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-mips@linux-mips.org, Russell King <linux@arm.linux.org.uk>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org, Peter Zijlstra <peterz@infradead.org>, Rusty Russell <rusty@rustcorp.com.au>
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On Thu, 27 Jun 2013 11:39:17 +0200 Ralf Baechle <ralf@linux-mips.org> wrote:

> Imho de7d2b567d040e3b67fe7121945982f14343213d [mm/vmalloc.c: report more
> vmalloc failures] is overly strict in that it also reports zero-sized
> allocations.  I consider such allocations stupid but legitimiate and often
> better preferrable over having to scatter checks for zero size all over
> place.  So maybe something like below patch?
> 
> ...
>
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1679,7 +1679,10 @@ void *__vmalloc_node_range(unsigned long size, 
> unsigned long align,
>       unsigned long real_size = size;
>  
>       size = PAGE_ALIGN(size);
> -     if (!size || (size >> PAGE_SHIFT) > totalram_pages)
> +     if (unlikely(!size))
> +             return NULL;
> +
> +     if ((size >> PAGE_SHIFT) > totalram_pages)
>               goto fail;
>  
>       area = __get_vm_area_node(size, align, VM_ALLOC | VM_UNLIST,
> @@ -1711,6 +1714,7 @@ fail:
>       warn_alloc_failed(gfp_mask, 0,
>                         "vmalloc: allocation failure: %lu bytes\n",
>                         real_size);
> +
>       return NULL;
>  }

If the caller actually dereferences the returned pointer the kernel
will go oops, which should provide adequate notification of a
programming error ;) But all callers should be checking the return
value.  So I worry about the by-far-most-common case where code does

        size = some_screwed_up_calculation();
        p = vmalloc(size);
        if (!p)
                return -ENOMEM;

So the mistake gets propagated back to who-knows-where as memory
exhaustion and thereby becomes a lot harder to diagnose.


How many callsites really truly need to be edited to avoid the warning?


Veli-Pekka's original patch would be neater if we were to add a new

void *__vmalloc_node_range_zero_size_ok(<args>)
{
        if (size == 0)
                return NULL;
        return __vmalloc_node_range(<args>);
}

(with a better name than __vmalloc_node_range_zero_size_ok!)

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