Analyzer of MongoDB 2.4 's new feature returned ununderstandable results. The value of "resident" totally different from "pagesInMemory". But why ? -"resident" are coming from STAT. -"pageInMemory" are coming from mincore(). This slide illustrates this issue.

1. Why mincore()
returns different
value of stat ?

2. mapped file

3. Interior of a process
User-land Kernel-land
virtual mem physical mem
mapped area
mmap() 1MB
mapped file
page 1
used by
page 2
page 3
FS others
page 4
page 5
page 6
page 7
page 256

4. User-land Kernel-land
virtual mem physical mem
mapped area
page 1
page 2
page 3
page 4
page 5
page 6
page 7
page 256
mapped file
page 1
used by
page 2
page 3
FS others
page 4
page 5
page 6
page 7
page 256

5. Major page fault
User-land Kernel-land
virtual mem physical mem
mapped area
touch page 1
page 2
page 3
page 4
page 5
page 6
page 7
page 256
mapped file
page 1
used by
page 2
page 3
FS others
page 4
page 5
page 6
page 7
page 256

6. Major page fault
User-land Kernel-land
virtual mem physical mem
mapped area
touch page 1 page 2
page 2
page 3
Read page from disk
page 4
page 5
page 6
page 7
page 256
mapped file
page 1
used by
page 2
page 3
FS others
page 4
page 5
page 6
page 7
page 256

7. Major page fault
User-land Kernel-land
virtual mem physical mem
mapped area Associate physical memory
with virtual memory
touch page 1 page 2
page 2
page 3
page 4
page 5
page 6
page 7
page 256
mapped file
page 1
used by
page 2
page 3
FS others
page 4
page 5
page 6
page 7
page 256

8. User-land Kernel-land
virtual mem physical mem
mapped area
page 1 page 2
touch page 2 page 3
page 3
page 4
page 5
page 6
page 7
page 256
mapped file
page 1
used by
page 2
page 3
FS others
page 4
page 5
page 6
page 7
page 256

9. User-land Kernel-land
virtual mem physical mem
mapped area
page 1 page 2
page 2 page 3
page 3 page 6
page 4 page 7
page 5 page 1
page 6 page 4
page 7
page 256
mapped file
page 1
used by
page 2
page 3
FS others
page 4
page 5
page 6
page 7
page 256

10. swap out (Just image , not actual)

11. User-land Kernel-land
virtual mem physical mem
mapped area
page 1 page 2
page 2 page 3
page 3 page 6
touch page 4 page 7
page 5 page 1
page 6 page 4
page 7
page 256
mapped file
page 1
used by
page 2
page 3
FS others
page 4
page 5
page 6
page 7
page 256

12. User-land Kernel-land
virtual mem physical mem
mapped area
page 1 page 2
page 2 page 3
page 3 page 6
touch page 4 page 7
page 5 page 1
page 6 page 4
page 7
page 256
mapped file
page 1
used by
page 2
page 3
FS others
page 4
page 5
page 6
page 7
page 256

13. User-land Kernel-land
virtual mem physical mem
mapped area
page 1
page 2 page 3
page 3 page 6
touch page 4 page 7
page 5 page 1
page 6 page 4
page 7
page 256
mapped file
page 1
used by
page 2
page 3
FS others
page 4
page 5
page 6
page 7
page 256

14. User-land Kernel-land
virtual mem physical mem
mapped area
page 1 page 5
page 2 page 3
page 3 page 6
touch page 4 page 7
page 5 page 1
page 6 page 4
page 7
page 256
mapped file
page 1
used by
page 2
page 3
FS others
page 4
page 5
page 6
page 7
page 256

15. restart the process

16. Old process
User-land Kernel-land
virtual mem physical mem
mapped area
page 1 page 5
page 2 page 3
page 3 page 6
page 4 page 7
page 5 page 1
page 6 page 4
page 7
page 256
mapped file
page 1
used by
page 2
page 3
FS others
page 4
page 5
page 6
page 7
page 256

17. Kill process
Nothing Kernel-land
physical mem
page 5
page 3
page 6
page 7
page 1
page 4
mapped file
page 1
used by
page 2
page 3
FS others
page 4
page 5
page 6
page 7
page 256

18. New process
User-land Kernel-land
virtual mem physical mem
page 5
page 3
page 6
page 7
page 1
page 4
mapped file
page 1
used by
page 2
page 3
FS others
page 4
page 5
page 6
page 7
page 256

19. User-land Kernel-land
virtual mem physical mem
mapped area
page 1 page 5
page 2 page 3
page 3 page 6
page 4 page 7
page 5 page 1
page 6 page 4
page 7
mmap() 1MB
page 256
mapped file
page 1
used by
page 2
page 3
FS others
page 4
page 5
page 6
page 7
page 256

20. User-land Kernel-land
virtual mem physical mem
mapped area
page 1
Actually, some pages are page 5
page 3
page 2
page 3 on memory page 6
page 4 page 7
page 5 page 1
page 6 page 4
page 7
page 256
mapped file
page 1
used by
page 2
page 3
FS others
page 4
page 5
page 6
page 7
page 256

21. Minor page fault
User-land Kernel-land
virtual mem physical mem
mapped area
ONLY associate physical
page 1 memory with virtual memory page 5
page 2 page 3
page 3 page 6
touch page 4 page 7
page 5 page 1
page 6 page 4
page 7
page 256
mapped file
page 1
used by
page 2
page 3
FS others
page 4
page 5
page 6
page 7
page 256

22. names & seeing

23. User-land Kernel-land
virtual mem physical mem
top
mapped area
/proc/<pid>/smaps
/proc/<pid>/statm page 1 PTE VMA page 5
: page 2 page 3
page 3 page 6
page 4 page 7
page 5 page 1
page 6 page 4
page 7
VIRT(virtual)
page 256
RES(resident) mapped file
page 1
used by
SHR page 2
FS others
page 3
page 4
page 5
mincore() page 6
page 7
page 256

24. Kernel code

25. fs/proc/task_mmu.c#L447 mm/mincore.c#L108
static void smaps_pte_entry(pte_t ptent, unsigned long addr, static void mincore_pte_range(struct vm_area_struct *vma,
unsigned long ptent_size, struct mm_walk pmd_t *pmd,
*walk) unsigned long addr, unsigned long end,
{ unsigned char *vec)
struct mem_size_stats *mss = walk->private; {
struct vm_area_struct *vma = mss->vma; unsigned long next;
pgoff_t pgoff = linear_page_index(vma, addr); spinlock_t *ptl;
struct page *page = NULL; pte_t *ptep;
int mapcount;
ptep = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
if (pte_present(ptent)) { do {
page = vm_normal_page(vma, addr, ptent); pte_t pte = *ptep;
} else if (is_swap_pte(ptent)) { pgoff_t pgoff;
swp_entry_t swpent = pte_to_swp_entry(ptent);
next = addr + PAGE_SIZE;
if (!non_swap_entry(swpent)) if (pte_none(pte))
mss->swap += ptent_size; mincore_unmapped_range(vma, addr, next, vec);
else if (is_migration_entry(swpent)) else if (pte_present(pte))
page = migration_entry_to_page(swpent); *vec = 1;
} else if (pte_file(ptent)) { else if (pte_file(pte)) {
if (pte_to_pgoff(ptent) != pgoff) pgoff = pte_to_pgoff(pte);
mss->nonlinear += ptent_size; *vec = mincore_page(vma->vm_file->f_mapping, pgoff);
} } else { /* pte is a swap entry */
swp_entry_t entry = pte_to_swp_entry(pte);
if (!page) :
return; :
if (PageAnon(page))
mss->anonymous += ptent_size;
if (page->index != pgoff)
mss->nonlinear += ptent_size;
mss->resident += ptent_size;
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