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[PATCH] x86: fix small LDT allocation for Xen

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  [PATCH] x86: fix small LDT allocation for Xen "Jan Beulich" <JBeulich@suse.com> - 2015-09-01 12:50 +0200
    Re: [PATCH] x86: fix small LDT allocation for Xen Andy Lutomirski <luto@amacapital.net> - 2015-09-01 21:40 +0200
      Re: [PATCH] x86: fix small LDT allocation for Xen "Jan Beulich" <JBeulich@suse.com> - 2015-09-02 09:00 +0200

#1216762 — [PATCH] x86: fix small LDT allocation for Xen

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@suse.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.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);
 
 	if (!new_ldt->entries) {
 		kfree(new_ldt);



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#1217042

On Tue, Sep 1, 2015 at 3:48 AM, Jan Beulich <JBeulich@suse.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@suse.com>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Cc: David Vrabel <david.vrabel@citrix.com>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.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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#1217356

>>> On 01.09.15 at 21:37, <luto@amacapital.net> wrote:
> On Tue, Sep 1, 2015 at 3:48 AM, Jan Beulich <JBeulich@suse.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@suse.com>
>> Cc: Andy Lutomirski <luto@kernel.org>
>> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
>> Cc: David Vrabel <david.vrabel@citrix.com>
>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.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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