[PATCH] statm: shared = rss - anon_rss

The third "shared" field of /proc/$pid/statm in 2.4 was a count of pages in
the mm whose page_count is more than 1 (oddly, including pages shared just
with swapcache).  That's too costly to calculate each time, so 2.6 changed
it to the total file-backed extent.  But Andrea knows apps and users
surprised when (rss - shared) goes negative: we need to provide an rss-like
statistic, close to the 2.4 interpretation.

Something that's quick and easy to maintain accurately is mm->anon_rss, the
count of anonymous pages in the mm.  Then shared = rss - anon_rss gives a
pretty good and meaningful approximation to 2.4's intention: wli confirms
that this will be useful to Oracle too.

Where to show it?  I think it's best to treat this as a bugfix and show it
in the third field of /proc/$pid/statm, after resident, as before - there's
no evidence that the total file-backed extent was found useful.

Albert would like other fields to revert to page counts, but that's a lot
harder: if mprotect can change the category of a page, then it can't be
accounted as simply as this.  Only go that route if real need shown.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Acked-by: William Irwin <wli@holomorphy.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/proc/task_mmu.c

@@ -37,7 +37,7 @@ unsigned long task_vsize(struct mm_struct *mm)
 int task_statm(struct mm_struct *mm, int *shared, int *text,
 	       int *data, int *resident)
 {
-	*shared = mm->shared_vm;
+	*shared = mm->rss - mm->anon_rss;
 	*text = (PAGE_ALIGN(mm->end_code) - (mm->start_code & PAGE_MASK))
 								>> PAGE_SHIFT;
 	*data = mm->total_vm - mm->shared_vm - *text;

include/linux/sched.h

@@ -216,7 +216,7 @@ struct mm_struct {
 	atomic_t mm_count;			/* How many references to "struct mm_struct" (users count as 1) */
 	int map_count;				/* number of VMAs */
 	struct rw_semaphore mmap_sem;
-	spinlock_t page_table_lock;		/* Protects task page tables and mm->rss */
+	spinlock_t page_table_lock;		/* Protects page tables, mm->rss, mm->anon_rss */
 
 	struct list_head mmlist;		/* List of maybe swapped mm's.  These are globally strung
 						 * together off init_mm.mmlist, and are protected
@@ -226,7 +226,7 @@ struct mm_struct {
 	unsigned long start_code, end_code, start_data, end_data;
 	unsigned long start_brk, brk, start_stack;
 	unsigned long arg_start, arg_end, env_start, env_end;
-	unsigned long rss, total_vm, locked_vm, shared_vm;
+	unsigned long rss, anon_rss, total_vm, locked_vm, shared_vm;
 	unsigned long exec_vm, stack_vm, reserved_vm, def_flags, nr_ptes;
 
 	unsigned long saved_auxv[42]; /* for /proc/PID/auxv */

kernel/fork.c

@@ -173,6 +173,7 @@ static inline int dup_mmap(struct mm_struct * mm, struct mm_struct * oldmm)
 	mm->free_area_cache = oldmm->mmap_base;
 	mm->map_count = 0;
 	mm->rss = 0;
+	mm->anon_rss = 0;
 	cpus_clear(mm->cpu_vm_mask);
 	mm->mm_rb = RB_ROOT;
 	rb_link = &mm->mm_rb.rb_node;

mm/memory.c

@@ -334,6 +334,8 @@ skip_copy_pmd_range:	address = (address + PGDIR_SIZE) & PGDIR_MASK;
 				pte = pte_mkold(pte);
 				get_page(page);
 				dst->rss++;
+				if (PageAnon(page))
+					dst->anon_rss++;
 				set_pte(dst_pte, pte);
 				page_dup_rmap(page);
 cont_copy_pte_range_noset:
@@ -424,7 +426,9 @@ static void zap_pte_range(struct mmu_gather *tlb,
 				set_pte(ptep, pgoff_to_pte(page->index));
 			if (pte_dirty(pte))
 				set_page_dirty(page);
-			if (pte_young(pte) && !PageAnon(page))
+			if (PageAnon(page))
+				tlb->mm->anon_rss--;
+			else if (pte_young(pte))
 				mark_page_accessed(page);
 			tlb->freed++;
 			page_remove_rmap(page);
@@ -1109,6 +1113,8 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct * vma,
 	spin_lock(&mm->page_table_lock);
 	page_table = pte_offset_map(pmd, address);
 	if (likely(pte_same(*page_table, pte))) {
+		if (PageAnon(old_page))
+			mm->anon_rss--;
 		if (PageReserved(old_page))
 			++mm->rss;
 		else

mm/rmap.c

@@ -432,6 +432,8 @@ void page_add_anon_rmap(struct page *page,
 	BUG_ON(PageReserved(page));
 	BUG_ON(!anon_vma);
 
+	vma->vm_mm->anon_rss++;
+
 	anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
 	index = (address - vma->vm_start) >> PAGE_SHIFT;
 	index += vma->vm_pgoff;
@@ -584,6 +586,7 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma)
 		}
 		set_pte(pte, swp_entry_to_pte(entry));
 		BUG_ON(pte_file(*pte));
+		mm->anon_rss--;
 	}
 
 	mm->rss--;