linux-imx

.. _split_page_table_lock:

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Split page table lock

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Originally, mm->page_table_lock spinlock protected all page tables of the

mm_struct. But this approach leads to poor page fault scalability of

multi-threaded applications due high contention on the lock. To improve

scalability, split page table lock was introduced.

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With split page table lock we have separate per-table lock to serialize

access to the table. At the moment we use split lock for PTE and PMD

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()

    maps pte and takes PTE table lock, returns pointer to the taken

    lock;

 - pte_unmap_unlock()

    unlocks and unmaps PTE table;

 - pte_alloc_map_lock()

    allocates PTE table if needed and take the lock, returns pointer

    to taken lock or NULL if allocation failed;

 - pte_lockptr()

    returns pointer to PTE table lock;

 - pmd_lock()

    takes PMD table lock, returns pointer to taken lock;

 - pmd_lockptr()

    returns pointer to PMD table lock;

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Split page table lock for PTE tables is enabled compile-time if

CONFIG_SPLIT_PTLOCK_CPUS (usually 4) is less or equal to NR_CPUS.

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

tables and the architecture supports it (see below).

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Hugetlb and split page table lock

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Hugetlb can support several page sizes. We use split lock only for PMD

level, but not for PUD.

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Hugetlb-specific helpers:

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 - huge_pte_lock()

    takes pmd split lock for PMD_SIZE page, mm->page_table_lock

    otherwise;

 - huge_pte_lockptr()

    returns pointer to table lock;

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Support of split page table lock by an architecture

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There's no need in special enabling of PTE split page table lock:

everything required is done by pgtable_page_ctor() and pgtable_page_dtor(),

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

allocation: slab uses page->slab_cache for its pages.

This field shares storage with page->ptl.