Re: [PATCH 2/3] mm/page_ref: Ensure page_ref_unfreeze is ordered against prior accesses

[Vlastimil Babka <vbabka@suse.cz>]

On 06/08/2017 12:34 PM, Will Deacon wrote:
> On Thu, Jun 08, 2017 at 11:38:21AM +0200, Vlastimil Babka wrote:
>>
>> Undecided if it's really needed. This is IMHO not the classical case
>> from Documentation/core-api/atomic_ops.rst where we have to make
>> modifications visible before we let others see them? Here the one who is
>> freezing is doing it so others can't get their page pin and interfere
>> with the freezer's work. But maybe there are some (documented or not)
>> consistency guarantees to expect once you obtain the pin, that can be
>> violated, or they might be added later, so it would be safer to add the
>> barrier?
> 
> The problem comes if the unfreeze is reordered so that it happens before the
> freezer has performed its work. For example, in
> migrate_huge_page_move_mapping:
> 
> 
> 	if (!page_ref_freeze(page, expected_count)) {
> 		spin_unlock_irq(&mapping->tree_lock);
> 		return -EAGAIN;
> 	}
> 
> 	newpage->index = page->index;
> 	newpage->mapping = page->mapping;
> 
> 	get_page(newpage);
> 
> 	radix_tree_replace_slot(&mapping->page_tree, pslot, newpage);
> 
> 	page_ref_unfreeze(page, expected_count - 1);
> 
> 
> then there's nothing stopping the CPU (and potentially the compiler) from
> reordering the unfreeze call so that it effectively becomes:
> 
> 
> 	if (!page_ref_freeze(page, expected_count)) {
> 		spin_unlock_irq(&mapping->tree_lock);
> 		return -EAGAIN;
> 	}
> 
> 	page_ref_unfreeze(page, expected_count - 1);
> 
> 	newpage->index = page->index;
> 	newpage->mapping = page->mapping;
> 
> 	get_page(newpage);
> 
> 	radix_tree_replace_slot(&mapping->page_tree, pslot, newpage);
> 
> 
> which then means that the freezer's work is carried out without the page
> being frozen.

But in this example the modifications are for newpage and freezing is
for page, so I think it doesn't apply. But I get the point.

> Will
> 
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