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author | Kirill A. Shutemov <kirill.shutemov@linux.intel.com> | 2014-06-04 16:10:55 -0700 |
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committer | Linus Torvalds <torvalds@linux-foundation.org> | 2014-06-04 16:54:12 -0700 |
commit | 1fdb412bd825998efbced3a16f6ce7e0329728cf (patch) | |
tree | f682bad282240bfb01b8f66ee2d9765915227e8d /mm/memory.c | |
parent | a9b0f8618d46ba027243b8ecb5c2468a7112d235 (diff) | |
download | linux-1fdb412bd825998efbced3a16f6ce7e0329728cf.tar.gz linux-1fdb412bd825998efbced3a16f6ce7e0329728cf.tar.bz2 linux-1fdb412bd825998efbced3a16f6ce7e0329728cf.zip |
mm: document do_fault_around() feature
Some clarification on how faultaround works.
[akpm@linux-foundation.org: tweak comment text]
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm/memory.c')
-rw-r--r-- | mm/memory.c | 27 |
1 files changed, 27 insertions, 0 deletions
diff --git a/mm/memory.c b/mm/memory.c index 62a08a7badc4..d67fd9fcf1f2 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -2760,6 +2760,10 @@ void do_set_pte(struct vm_area_struct *vma, unsigned long address, static unsigned long fault_around_bytes = 65536; +/* + * fault_around_pages() and fault_around_mask() round down fault_around_bytes + * to nearest page order. It's what do_fault_around() expects to see. + */ static inline unsigned long fault_around_pages(void) { return rounddown_pow_of_two(fault_around_bytes) / PAGE_SIZE; @@ -2801,6 +2805,29 @@ static int __init fault_around_debugfs(void) late_initcall(fault_around_debugfs); #endif +/* + * do_fault_around() tries to map few pages around the fault address. The hope + * is that the pages will be needed soon and this will lower the number of + * faults to handle. + * + * It uses vm_ops->map_pages() to map the pages, which skips the page if it's + * not ready to be mapped: not up-to-date, locked, etc. + * + * This function is called with the page table lock taken. In the split ptlock + * case the page table lock only protects only those entries which belong to + * the page table corresponding to the fault address. + * + * This function doesn't cross the VMA boundaries, in order to call map_pages() + * only once. + * + * fault_around_pages() defines how many pages we'll try to map. + * do_fault_around() expects it to return a power of two less than or equal to + * PTRS_PER_PTE. + * + * The virtual address of the area that we map is naturally aligned to the + * fault_around_pages() value (and therefore to page order). This way it's + * easier to guarantee that we don't cross page table boundaries. + */ static void do_fault_around(struct vm_area_struct *vma, unsigned long address, pte_t *pte, pgoff_t pgoff, unsigned int flags) { |