Commit 1fdb412b authored by Kirill A. Shutemov's avatar Kirill A. Shutemov Committed by Linus Torvalds

mm: document do_fault_around() feature

Some clarification on how faultaround works.

[ tweak comment text]
Signed-off-by: default avatarKirill A. Shutemov <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent a9b0f861
......@@ -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)
* 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
* 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)
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