linux-kernel - Re: [PATCH] x86: fix small LDT allocation for Xen

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Date:	Tue, 1 Sep 2015 12:37:58 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Andrew Lutomirski <luto@...nel.org>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: fix small LDT allocation for Xen

On Tue, Sep 1, 2015 at 3:48 AM, Jan Beulich <JBeulich@...e.com> wrote:
> While commit 37868fe113 ("x86/ldt: Make modify_ldt synchronous") added
> a nice comment explaining that Xen needs page-aligned whole page chunks
> for guest descriptor tables, it then nevertheless used kzalloc() on the
> small size path. As I'm unaware of guarantees for kmalloc(PAGE_SIZE, )
> to return page-aligned memory blocks, I believe this needs to be
> switched back to __get_free_page().
>
> Signed-off-by: Jan Beulich <jbeulich@...e.com>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> Cc: David Vrabel <david.vrabel@...rix.com>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> ---
>  arch/x86/kernel/ldt.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> --- 4.2/arch/x86/kernel/ldt.c
> +++ 4.2-x86-LDT-alloc/arch/x86/kernel/ldt.c
> @@ -58,7 +58,7 @@ static struct ldt_struct *alloc_ldt_stru
>         if (alloc_size > PAGE_SIZE)
>                 new_ldt->entries = vzalloc(alloc_size);
>         else
> -               new_ldt->entries = kzalloc(PAGE_SIZE, GFP_KERNEL);
> +               new_ldt->entries = (void *)__get_free_page(GFP_KERNEL|__GFP_ZERO);

This would need a corresponding change to the kfree path, right?

--Andy
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