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authorKirill A. Shutemov <kirill.shutemov@linux.intel.com>2014-06-04 16:10:55 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2014-06-04 16:54:12 -0700
commit1fdb412bd825998efbced3a16f6ce7e0329728cf (patch)
treef682bad282240bfb01b8f66ee2d9765915227e8d /mm/memory.c
parenta9b0f8618d46ba027243b8ecb5c2468a7112d235 (diff)
downloadlwn-1fdb412bd825998efbced3a16f6ce7e0329728cf.tar.gz
lwn-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.c27
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)
{