[PATCH] mm: update_hiwaters just in time

update_mem_hiwater has attracted various criticisms, in particular from those
concerned with mm scalability.  Originally it was called whenever rss or
total_vm got raised.  Then many of those callsites were replaced by a timer
tick call from account_system_time.  Now Frank van Maarseveen reports that to
be found inadequate.  How about this?  Works for Frank.

Replace update_mem_hiwater, a poor combination of two unrelated ops, by macros
update_hiwater_rss and update_hiwater_vm.  Don't attempt to keep
mm->hiwater_rss up to date at timer tick, nor every time we raise rss (usually
by 1): those are hot paths.  Do the opposite, update only when about to lower
rss (usually by many), or just before final accounting in do_exit.  Handle
mm->hiwater_vm in the same way, though it's much less of an issue.  Demand
that whoever collects these hiwater statistics do the work of taking the
maximum with rss or total_vm.

And there has been no collector of these hiwater statistics in the tree.  The
new convention needs an example, so match Frank's usage by adding a VmPeak
line above VmSize to /proc/<pid>/status, and also a VmHWM line above VmRSS
(High-Water-Mark or High-Water-Memory).

There was a particular anomaly during mremap move, that hiwater_vm might be
captured too high.  A fleeting such anomaly remains, but it's quickly
corrected now, whereas before it would stick.

What locking?  None: if the app is racy then these statistics will be racy,
it's not worth any overhead to make them exact.  But whenever it suits,
hiwater_vm is updated under exclusive mmap_sem, and hiwater_rss under
page_table_lock (for now) or with preemption disabled (later on): without
going to any trouble, minimize the time between reading current values and
updating, to minimize those occasions when a racing thread bumps a count up
and back down in between.

Signed-off-by: Hugh Dickins <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/fremap.c

@@ -143,8 +143,10 @@ int install_file_pte(struct mm_struct *mm, struct vm_area_struct *vma,
 	if (!pte)
 		goto err_unlock;
 
-	if (!pte_none(*pte) && zap_pte(mm, vma, addr, pte))
+	if (!pte_none(*pte) && zap_pte(mm, vma, addr, pte)) {
+		update_hiwater_rss(mm);
 		dec_mm_counter(mm, file_rss);
+	}
 
 	set_pte_at(mm, addr, pte, pgoff_to_pte(pgoff));
 	pte_val = *pte;