Knowing the portion of memory that is not used by a certain application
or memory cgroup (idle memory) can be useful for partitioning the system
efficiently, e.g. by setting memory cgroup limits appropriately.
Currently, the only means to estimate the amount of idle memory provided
by the kernel is /proc/PID/{clear_refs,smaps}: the user can clear the
access bit for all pages mapped to a particular process by writing 1 to
clear_refs, wait for some time, and then count smaps:Referenced.
However, this method has two serious shortcomings:

 - it does not count unmapped file pages
 - it affects the reclaimer logic

To overcome these drawbacks, this patch introduces two new page flags,
Idle and Young, and a new proc file, /proc/kpageidle. A page's Idle flag
can only be set from userspace by writing 1 to /proc/kpageidle at the
offset corresponding to the page, and it is cleared whenever the page is
accessed either through page tables (it is cleared in page_referenced()
in this case) or using the read(2) system call (mark_page_accessed()).
Thus by setting the Idle flag for pages of a particular workload, which
can be found e.g. by reading /proc/PID/pagemap, waiting for some time to
let the workload access its working set, and then reading the kpageidle
file, one can estimate the amount of pages that are not used by the
workload.

The Young page flag is used to avoid interference with the memory
reclaimer. A page's Young flag is set whenever the Access bit of a page
table entry pointing to the page is cleared by writing to kpageidle. If
page_referenced() is called on a Young page, it will add 1 to its return
value, therefore concealing the fact that the Access bit was cleared.

Note, since there is no room for extra page flags on 32 bit, this
feature uses extended page flags when compiled on 32 bit.

Signed-off-by: Vladimir Davydov <redacted>
---
 Documentation/vm/pagemap.txt |   10 ++-
 fs/proc/page.c               |  154 ++++++++++++++++++++++++++++++++++++++++++
 fs/proc/task_mmu.c           |    4 +-
 include/linux/mm.h           |   88 ++++++++++++++++++++++++
 include/linux/page-flags.h   |    9 +++
 include/linux/page_ext.h     |    4 ++
 mm/Kconfig                   |   12 ++++
 mm/debug.c                   |    4 ++
 mm/page_ext.c                |    3 +
 mm/rmap.c                    |    7 ++
 mm/swap.c                    |    2 +
 11 files changed, 295 insertions(+), 2 deletions(-)

diff --git a/Documentation/vm/pagemap.txt b/Documentation/vm/pagemap.txt
index a9b7afc8fbc6..ac6fd32a9296 100644
--- a/Documentation/vm/pagemap.txt
+++ b/Documentation/vm/pagemap.txt

@@ -5,7 +5,7 @@ pagemap is a new (as of 2.6.25) set of interfaces in the kernel that allow
 userspace programs to examine the page tables and related information by
 reading files in /proc.
 
-There are four components to pagemap:
+There are five components to pagemap:
 
  * /proc/pid/pagemap.  This file lets a userspace process find out which
    physical frame each virtual page is mapped to.  It contains one 64-bit

@@ -69,6 +69,14 @@ There are four components to pagemap:
    memory cgroup each page is charged to, indexed by PFN. Only available when
    CONFIG_MEMCG is set.
 
+ * /proc/kpageidle.  For each page this file contains a 64-bit number, which
+   equals 1 if the page is idle or 0 otherwise, indexed by PFN. A page is
+   considered idle if it has not been accessed since it was marked idle. To
+   mark a page idle one should write 1 to this file at the offset corresponding
+   to the page. Only user memory pages can be marked idle, for other page types
+   input is silently ignored. Writing to this file beyond max PFN results in
+   the ENXIO error. Only available when CONFIG_IDLE_PAGE_TRACKING is set.
+