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![Kirill A. Shutemov](/assets/img/avatar_default.png)
Hugh has pointed that compound_head() call can be unsafe in some context. There's one example: CPU0 CPU1 isolate_migratepages_block() page_count() compound_head() !!PageTail() == true put_page() tail->first_page = NULL head = tail->first_page alloc_pages(__GFP_COMP) prep_compound_page() tail->first_page = head __SetPageTail(p); !!PageTail() == true <head == NULL dereferencing> The race is pure theoretical. I don't it's possible to trigger it in practice. But who knows. We can fix the race by changing how encode PageTail() and compound_head() within struct page to be able to update them in one shot. The patch introduces page->compound_head into third double word block in front of compound_dtor and compound_order. Bit 0 encodes PageTail() and the rest bits are pointer to head page if bit zero is set. The patch moves page->pmd_huge_pte out of word, just in case if an architecture defines pgtable_t into something what can have the bit 0 set. hugetlb_cgroup uses page->lru.next in the second tail page to store pointer struct hugetlb_cgroup. The patch switch it to use page->private in the second tail page instead. The space is free since ->first_page is removed from the union. The patch also opens possibility to remove HUGETLB_CGROUP_MIN_ORDER limitation, since there's now space in first tail page to store struct hugetlb_cgroup pointer. But that's out of scope of the patch. That means page->compound_head shares storage space with: - page->lru.next; - page->next; - page->rcu_head.next; That's too long list to be absolutely sure, but looks like nobody uses bit 0 of the word. page->rcu_head.next guaranteed[1] to have bit 0 clean as long as we use call_rcu(), call_rcu_bh(), call_rcu_sched(), or call_srcu(). But future call_rcu_lazy() is not allowed as it makes use of the bit and we can get false positive PageTail(). [1] http://lkml.kernel.org/g/20150827163634.GD4029@linux.vnet.ibm.com Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Acked-by: Michal Hocko <mhocko@suse.com> Reviewed-by: Andrea Arcangeli <aarcange@redhat.com> Cc: Hugh Dickins <hughd@google.com> Cc: David Rientjes <rientjes@google.com> Cc: Vlastimil Babka <vbabka@suse.cz> Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com> Cc: Andi Kleen <ak@linux.intel.com> Cc: Christoph Lameter <cl@linux.com> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com> Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
94 lines
3.5 KiB
Text
94 lines
3.5 KiB
Text
Split page table lock
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=====================
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Originally, mm->page_table_lock spinlock protected all page tables of the
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mm_struct. But this approach leads to poor page fault scalability of
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multi-threaded applications due high contention on the lock. To improve
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scalability, split page table lock was introduced.
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With split page table lock we have separate per-table lock to serialize
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access to the table. At the moment we use split lock for PTE and PMD
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tables. Access to higher level tables protected by mm->page_table_lock.
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There are helpers to lock/unlock a table and other accessor functions:
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- pte_offset_map_lock()
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maps pte and takes PTE table lock, returns pointer to the taken
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lock;
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- pte_unmap_unlock()
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unlocks and unmaps PTE table;
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- pte_alloc_map_lock()
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allocates PTE table if needed and take the lock, returns pointer
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to taken lock or NULL if allocation failed;
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- pte_lockptr()
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returns pointer to PTE table lock;
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- pmd_lock()
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takes PMD table lock, returns pointer to taken lock;
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- pmd_lockptr()
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returns pointer to PMD table lock;
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Split page table lock for PTE tables is enabled compile-time if
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CONFIG_SPLIT_PTLOCK_CPUS (usually 4) is less or equal to NR_CPUS.
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If split lock is disabled, all tables guaded by mm->page_table_lock.
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Split page table lock for PMD tables is enabled, if it's enabled for PTE
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tables and the architecture supports it (see below).
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Hugetlb and split page table lock
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---------------------------------
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Hugetlb can support several page sizes. We use split lock only for PMD
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level, but not for PUD.
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Hugetlb-specific helpers:
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- huge_pte_lock()
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takes pmd split lock for PMD_SIZE page, mm->page_table_lock
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otherwise;
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- huge_pte_lockptr()
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returns pointer to table lock;
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Support of split page table lock by an architecture
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---------------------------------------------------
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There's no need in special enabling of PTE split page table lock:
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everything required is done by pgtable_page_ctor() and pgtable_page_dtor(),
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which must be called on PTE table allocation / freeing.
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Make sure the architecture doesn't use slab allocator for page table
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allocation: slab uses page->slab_cache for its pages.
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This field shares storage with page->ptl.
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PMD split lock only makes sense if you have more than two page table
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levels.
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PMD split lock enabling requires pgtable_pmd_page_ctor() call on PMD table
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allocation and pgtable_pmd_page_dtor() on freeing.
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Allocation usually happens in pmd_alloc_one(), freeing in pmd_free() and
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pmd_free_tlb(), but make sure you cover all PMD table allocation / freeing
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paths: i.e X86_PAE preallocate few PMDs on pgd_alloc().
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With everything in place you can set CONFIG_ARCH_ENABLE_SPLIT_PMD_PTLOCK.
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NOTE: pgtable_page_ctor() and pgtable_pmd_page_ctor() can fail -- it must
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be handled properly.
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page->ptl
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---------
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page->ptl is used to access split page table lock, where 'page' is struct
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page of page containing the table. It shares storage with page->private
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(and few other fields in union).
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To avoid increasing size of struct page and have best performance, we use a
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trick:
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- if spinlock_t fits into long, we use page->ptr as spinlock, so we
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can avoid indirect access and save a cache line.
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- if size of spinlock_t is bigger then size of long, we use page->ptl as
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pointer to spinlock_t and allocate it dynamically. This allows to use
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split lock with enabled DEBUG_SPINLOCK or DEBUG_LOCK_ALLOC, but costs
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one more cache line for indirect access;
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The spinlock_t allocated in pgtable_page_ctor() for PTE table and in
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pgtable_pmd_page_ctor() for PMD table.
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Please, never access page->ptl directly -- use appropriate helper.
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