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mm: document do_fault_around() feature

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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>
This commit is contained in:
Kirill A. Shutemov 2014-06-04 16:10:55 -07:00 committed by Linus Torvalds
parent a9b0f8618d
commit 1fdb412bd8
1 changed files with 27 additions and 0 deletions
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27
mm/memory.c
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@ -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)
{