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

mm: document do_fault_around() feature 



Some clarification on how faultaround works.

[akpm@linux-foundation.org: tweak comment text]
Signed-off-by: default avatarKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent a9b0f861

mm/memory.c

+27 −0
Original line number Diff line number Diff line
@@ -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)

{