linux-kernel - Re: [PATCH 01/13] AppArmor: misc. base functions and defines

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Message-ID: <AANLkTikk3tNM-fsXuS_SeAWgaYYCFrFAToRXOBkPxwy9@mail.gmail.com>
Date:	Fri, 30 Jul 2010 12:20:27 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	John Johansen <john.johansen@...onical.com>
Cc:	linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	Nick Piggin <npiggin@...e.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>, xiaosuo@...il.com,
	laijs@...fujitsu.com
Subject: Re: [PATCH 01/13] AppArmor: misc. base functions and defines

On Fri, Jul 30, 2010 at 12:47 AM, John Johansen
<john.johansen@...onical.com> wrote:
> +/**
> + * kvmalloc - do allocation preferring kmalloc but falling back to vmalloc
> + * @size: size of allocation
> + *
> + * Return: allocated buffer or NULL if failed
> + *
> + * It is possible that policy being loaded from the user is larger than
> + * what can be allocated by kmalloc, in those cases fall back to vmalloc.
> + */
> +void *kvmalloc(size_t size)
> +{
> +       void *buffer = NULL;
> +
> +       if (size == 0)
> +               return NULL;
> +
> +       /* do not attempt kmalloc if we need more than 16 pages at once */
> +       if (size <= (16*PAGE_SIZE))
> +               buffer = kmalloc(size, GFP_NOIO | __GFP_NOWARN);

16 pages is a lot of memory for 64 K pages. What's the purpose of
GFP_NOIO here? vmalloc() will do GFP_KERNEL allocations anyway.

> +       if (!buffer) {
> +               /* see kvfree for why size must be at least work_struct size
> +                * when allocated via vmalloc
> +                */
> +               if (size < sizeof(struct work_struct))
> +                       size = sizeof(struct work_struct);
> +               buffer = vmalloc(size);
> +       }
> +       return buffer;
> +}

Please don't hide this into apparmor internals. People have invented
this function in the past so maybe it's time to put it in mm/util.c?

> +
> +/**
> + * do_vfree - workqueue routine for freeing vmalloced memory
> + * @work: data to be freed
> + *
> + * The work_struct is overlaid to the data being freed, as at the point
> + * the work is scheduled the data is no longer valid, be its freeing
> + * needs to be delayed until safe.
> + */
> +static void do_vfree(struct work_struct *work)
> +{
> +       vfree(work);
> +}
> +
> +/**
> + * kvfree - free an allocation do by kvmalloc
> + * @buffer: buffer to free (MAYBE_NULL)
> + *
> + * Free a buffer allocated by kvmalloc
> + */
> +void kvfree(void *buffer)
> +{
> +       if (is_vmalloc_addr(buffer)) {
> +               /* Data is no longer valid so just use the allocated space
> +                * as the work_struct
> +                */
> +               struct work_struct *work = (struct work_struct *) buffer;
> +               INIT_WORK(work, do_vfree);
> +               schedule_work(work);

I don't understand this part here. Is it needed for interrupt contexts
or does vfree() sleep somewhere? If it's for the former, I think we
can just add a comment saying that kvmalloc/kvfree is not safe from
interrupt context and remove the schedule_work() parts here.

> +       } else
> +               kfree(buffer);
> +}
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