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b4ed71f557
The naming of pgtable_page_{ctor,dtor}() seems to have confused a few
people, and until recently arm64 used these erroneously/pointlessly for
other levels of page table.
To make it incredibly clear that these only apply to the PTE level, and to
align with the naming of pgtable_pmd_page_{ctor,dtor}(), let's rename them
to pgtable_pte_page_{ctor,dtor}().
These changes were generated with the following shell script:
----
git grep -lw 'pgtable_page_.tor' | while read FILE; do
sed -i '{s/pgtable_page_ctor/pgtable_pte_page_ctor/}' $FILE;
sed -i '{s/pgtable_page_dtor/pgtable_pte_page_dtor/}' $FILE;
done
----
... with the documentation re-flowed to remain under 80 columns, and
whitespace fixed up in macros to keep backslashes aligned.
There should be no functional change as a result of this patch.
Link: http://lkml.kernel.org/r/20190722141133.3116-1-mark.rutland@arm.com
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org> [m68k]
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
101 lines
3.6 KiB
ReStructuredText
101 lines
3.6 KiB
ReStructuredText
.. _split_page_table_lock:
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=====================
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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: everything
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required is done by pgtable_pte_page_ctor() and pgtable_pte_page_dtor(), which
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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_pte_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_pte_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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