Re: [PATCH 07/12] x86/virt/guest/xen: Remove use of pgd_list from the Xen guest code

[David Vrabel <david.vrabel@citrix.com>]

On 15/06/15 10:05, Ian Campbell wrote:
> On Sat, 2015-06-13 at 11:49 +0200, Ingo Molnar wrote:
>> xen_mm_pin_all()/unpin_all() are used to implement full guest instance
>> suspend/restore. It's a stop-all method that needs to iterate through
>> all allocated pgds in the system to fix them up for Xen's use.
>>
>> This code uses pgd_list, probably because it was an easy interface.
>>
>> But we want to remove the pgd_list, so convert the code over to walk
>> all tasks in the system. This is an equivalent method.

It is not equivalent because pgd_alloc() now populates entries in pgds
that are not visible to xen_mm_pin_all() (note how the original code
adds the pgd to the pgd_list in pgd_ctor() before calling
pgd_prepopulate_pmd()).  These newly allocated page tables won't be
correctly converted on suspend/resume and the new process will die after
resume.

David

>>
>> (As I don't use Xen this is was only build tested.)
> 
> In which case it seems extra important to copy the appropriate
> maintainers, which I've done here.
> 
> Ian.
> 
>>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Andy Lutomirski <luto@amacapital.net>
>> Cc: Borislav Petkov <bp@alien8.de>
>> Cc: Brian Gerst <brgerst@gmail.com>
>> Cc: Denys Vlasenko <dvlasenk@redhat.com>
>> Cc: H. Peter Anvin <hpa@zytor.com>
>> Cc: Linus Torvalds <torvalds@linux-foundation.org>
>> Cc: Oleg Nesterov <oleg@redhat.com>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Waiman Long <Waiman.Long@hp.com>
>> Cc: linux-mm@kvack.org
>> Signed-off-by: Ingo Molnar <mingo@kernel.org>
>> ---
>>  arch/x86/xen/mmu.c | 51 ++++++++++++++++++++++++++++++++++++++-------------
>>  1 file changed, 38 insertions(+), 13 deletions(-)
>>
>> diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
>> index dd151b2045b0..70a3df5b0b54 100644
>> --- a/arch/x86/xen/mmu.c
>> +++ b/arch/x86/xen/mmu.c
>> @@ -853,15 +853,27 @@ static void xen_pgd_pin(struct mm_struct *mm)
>>   */
>>  void xen_mm_pin_all(void)
>>  {
>> -	struct page *page;
>> +	struct task_struct *g, *p;
>>  
>> -	spin_lock(&pgd_lock);
>> +	spin_lock(&pgd_lock); /* Implies rcu_read_lock() for the task list iteration: */
>>  
>> -	list_for_each_entry(page, &pgd_list, lru) {
>> -		if (!PagePinned(page)) {
>> -			__xen_pgd_pin(&init_mm, (pgd_t *)page_address(page));
>> -			SetPageSavePinned(page);
>> +	for_each_process_thread(g, p) {
>> +		struct mm_struct *mm;
>> +		struct page *page;
>> +		pgd_t *pgd;
>> +
>> +		task_lock(p);
>> +		mm = p->mm;
>> +		if (mm) {
>> +			pgd = mm->pgd;
>> +			page = virt_to_page(pgd);
>> +
>> +			if (!PagePinned(page)) {
>> +				__xen_pgd_pin(&init_mm, pgd);
>> +				SetPageSavePinned(page);
>> +			}
>>  		}
>> +		task_unlock(p);
>>  	}
>>  
>>  	spin_unlock(&pgd_lock);
>> @@ -967,19 +979,32 @@ static void xen_pgd_unpin(struct mm_struct *mm)
>>   */
>>  void xen_mm_unpin_all(void)
>>  {
>> -	struct page *page;
>> +	struct task_struct *g, *p;
>>  
>> -	spin_lock(&pgd_lock);
>> +	spin_lock(&pgd_lock); /* Implies rcu_read_lock() for the task list iteration: */
>>  
>> -	list_for_each_entry(page, &pgd_list, lru) {
>> -		if (PageSavePinned(page)) {
>> -			BUG_ON(!PagePinned(page));
>> -			__xen_pgd_unpin(&init_mm, (pgd_t *)page_address(page));
>> -			ClearPageSavePinned(page);
>> +	for_each_process_thread(g, p) {
>> +		struct mm_struct *mm;
>> +		struct page *page;
>> +		pgd_t *pgd;
>> +
>> +		task_lock(p);
>> +		mm = p->mm;
>> +		if (mm) {
>> +			pgd = mm->pgd;
>> +			page = virt_to_page(pgd);
>> +
>> +			if (PageSavePinned(page)) {
>> +				BUG_ON(!PagePinned(page));
>> +				__xen_pgd_unpin(&init_mm, pgd);
>> +				ClearPageSavePinned(page);
>> +			}
>>  		}
>> +		task_unlock(p);
>>  	}
>>  
>>  	spin_unlock(&pgd_lock);
>> +	rcu_read_unlock();
>>  }
>>  
>>  static void xen_activate_mm(struct mm_struct *prev, struct mm_struct *next)
> 
> 

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