linux-kernel - Re: [PATCH v2] x86: fix small LDT allocation for Xen

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Date:	Wed, 02 Sep 2015 09:40:13 -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>,
	"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 v2] x86: fix small LDT allocation for Xen

>>> On 02.09.15 at 16:08, <luto@...capital.net> wrote:
> On Sep 2, 2015 5:46 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>
>> ---
>> v2: Also adjust the freeing side.
>> ---
>>  arch/x86/kernel/ldt.c |    4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> --- 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);
>> @@ -95,7 +95,7 @@ static void free_ldt_struct(struct ldt_s
>>         if (ldt->size * LDT_ENTRY_SIZE > PAGE_SIZE)
>>                 vfree(ldt->entries);
>>         else
>> -               kfree(ldt->entries);
>> +               put_page(virt_to_page(ldt->entries));
> 
> FWIW, I'm not convinced this is or was correct.  Using free_page looks
> a bit safer, and free_page does more than just put_page.

Actually I agree. put_page() is meant to be paired with get_page();
__get_free_pages() is just misleading (i.e. doesn't imply a get_page())
and instead is to be paired with free_pages(). Will do a v3 then.

Jan

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