Messages in this thread | | | Date | Thu, 8 Jun 2017 11:34:02 +0100 | From | Will Deacon <> | Subject | Re: [PATCH 2/3] mm/page_ref: Ensure page_ref_unfreeze is ordered against prior accesses |
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On Thu, Jun 08, 2017 at 11:38:21AM +0200, Vlastimil Babka wrote: > On 06/06/2017 07:58 PM, Will Deacon wrote: > > page_ref_freeze and page_ref_unfreeze are designed to be used as a pair, > > wrapping a critical section where struct pages can be modified without > > having to worry about consistency for a concurrent fast-GUP. > > > > Whilst page_ref_freeze has full barrier semantics due to its use of > > atomic_cmpxchg, page_ref_unfreeze is implemented using atomic_set, which > > doesn't provide any barrier semantics and allows the operation to be > > reordered with respect to page modifications in the critical section. > > > > This patch ensures that page_ref_unfreeze is ordered after any critical > > section updates, by invoking smp_mb__before_atomic() prior to the > > atomic_set. > > > > Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> > > Acked-by: Steve Capper <steve.capper@arm.com> > > Signed-off-by: Will Deacon <will.deacon@arm.com> > > 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.
Will
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