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

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Message-Id: <55E6B87E020000780009ED09@prv-mh.provo.novell.com>
Date:	Wed, 02 Sep 2015 00:51:10 -0600
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Andy Lutomirski" <luto@...capital.net>
Cc:	"David Vrabel" <david.vrabel@...rix.com>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Andrew Lutomirski" <luto@...nel.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86: fix small LDT allocation for Xen

>>> On 01.09.15 at 21:37, <luto@...capital.net> wrote:
> 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?

Oops - yes of course.

Jan

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