[PATCH v16 0/9] mm / virtio: Provide support for free page reporting

Alexander Duyck
2020-01-03T21:16:21Z

This series provides an asynchronous means of reporting free guest pages
to a hypervisor so that the memory associated with those pages can be
dropped and reused by other processes and/or guests on the host. Using
this it is possible to avoid unnecessary I/O to disk and greatly improve
performance in the case of memory overcommit on the host.

When enabled we will be performing a scan of free memory every 2 seconds
while pages of sufficiently high order are being freed. In each pass at
least one sixteenth of each free list will be reported. By doing this we
avoid racing against other threads that may be causing a high amount of
memory churn.

The lowest page order currently scanned when reporting pages is
pageblock_order so that this feature will not interfere with the use of
Transparent Huge Pages in the case of virtualization.

Currently this is only in use by virtio-balloon however there is the hope
that at some point in the future other hypervisors might be able to make
use of it. In the virtio-balloon/QEMU implementation the hypervisor is
currently using MADV_DONTNEED to indicate to the host kernel that the page
is currently free. It will be zeroed and faulted back into the guest the
next time the page is accessed.

To track if a page is reported or not the Uptodate flag was repurposed and
used as a Reported flag for Buddy pages. We walk though the free list
isolating pages and adding them to the scatterlist until we either
encounter the end of the list, processed as many pages as were listed in
nr_free prior to us starting, or have filled the scatterlist with pages to
be reported. If we fill the scatterlist before we reach the end of the
list we rotate the list so that the first unreported page we encounter is
moved to the head of the list as that is where we will resume after we
have freed the reported pages back into the tail of the list.

Below are the results from various benchmarks. I primarily focused on two
tests. The first is the will-it-scale/page_fault2 test, and the other is
a modified version of will-it-scale/page_fault1 that was enabled to use
THP. I did this as it allows for better visibility into different parts
of the memory subsystem. The guest is running with 32G for RAM on one
node of a E5-2630 v3. The host has had some features such as CPU turbo
disabled in the BIOS.

Test page_fault1 (THP) page_fault2
Name tasks Process Iter STDEV Process Iter STDEV
Baseline 1 1012402.50 0.14% 361855.25 0.81%
16 8827457.25 0.09% 3282347.00 0.34%

Patches Applied 1 1007897.00 0.23% 361887.00 0.26%
16 8784741.75 0.39% 3240669.25 0.48%

Patches Enabled 1 1010227.50 0.39% 359749.25 0.56%
16 8756219.00 0.24% 3226608.75 0.97%

Patches Enabled 1 1050982.00 4.26% 357966.25 0.14%
page shuffle 16 8672601.25 0.49% 3223177.75 0.40%

Patches Enabled 1 1003238.00 0.22% 360211.00 0.22%
shuffle w/ RFC 16 8767010.50 0.32% 3199874.00 0.71%

The results above are for a baseline with a linux-next-20191219 kernel,
that kernel with this patch set applied but page reporting disabled in
virtio-balloon, the patches applied and page reporting fully enabled, the
patches enabled with page shuffling enabled, and the patches applied with
page shuffling enabled and an RFC patch that makes used of MADV_FREE in
QEMU. These results include the deviation seen between the average value
reported here versus the high and/or low value. I observed that during the
test memory usage for the first three tests never dropped whereas with the
patches fully enabled the VM would drop to using only a few GB of the
host's memory when switching from memhog to page fault tests.

Any of the overhead visible with this patch set enabled seems due to page
faults caused by accessing the reported pages and the host zeroing the page
before giving it back to the guest. This overhead is much more visible when
using THP than with standard 4K pages. In addition page shuffling seemed to
increase the amount of faults generated due to an increase in memory churn.
The overhead is reduced when using MADV_FREE as we can avoid the extra
zeroing of the pages when they are reintroduced to the host, as can be seen
when the RFC is applied with shuffling enabled.

The overall guest size is kept fairly small to only a few GB while the test
is running. If the host memory were oversubscribed this patch set should
result in a performance improvement as swapping memory in the host can be
avoided.

A brief history on the background of free page reporting can be found at:
https://lore.kernel.org/lkml/29f43d5796feed0dec8e8bb98b187d9dac03b900.camel@linux.intel.com/

Changes from v14:
https://lore.kernel.org/lkml/20191119214454.24996.66289.stgit@localhost.localdomain/
Renamed "unused page reporting" to "free page reporting"
Updated code, kconfig, and patch descriptions
Split out patch for __free_isolated_page
Renamed function to __putback_isolated_page
Rewrote core reporting functionality
Added logic to reschedule worker in 2 seconds instead of run to completion
Removed reported_pages statistics
Removed REPORTING_REQUESTED bit used in zone flags
Replaced page_reporting_dev_info refcount with state variable
Removed scatterlist from page_reporting_dev_info
Removed capacity from page reporting device
Added dynamic scatterlist allocation/free at start/end of reporting process
Updated __free_one_page so that reported pages are not always added to tail
Added logic to handle error from report function
Updated virtio-balloon patch that adds support for page reporting
Updated patch description to try and highlight differences in approaches
Updated logic to reflect that we cannot limit the scatterlist from device
Added logic to return error from report function
Moved documentation patch to end of patch set

Changes from v15:
https://lore.kernel.org/lkml/20191205161928.19548.41654.stgit@localhost.localdomain/
Rebased on linux-next-20191219
Split out patches for budget and moving head to last page processed
Updated budget code to reduce how much memory is reported per pass
Added logic to also rotate the list if we exit due a page isolation failure
Added migratetype as argument in __putback_isolated_page

---

Alexander Duyck (9):
mm: Adjust shuffle code to allow for future coalescing
mm: Use zone and order instead of free area in free_list manipulators
mm: Add function __putback_isolated_page
mm: Introduce Reported pages
virtio-balloon: Pull page poisoning config out of free page hinting
virtio-balloon: Add support for providing free page reports to host
mm: Rotate free list so reported pages are moved to the tail of the list
mm: Add budget limit to how many pages can be reported per list per pass
mm: Add free page reporting documentation

Documentation/vm/free_page_reporting.rst | 41 +++
drivers/virtio/Kconfig | 1
drivers/virtio/virtio_balloon.c | 87 +++++++
include/linux/mmzone.h | 44 ----
include/linux/page-flags.h | 11 +
include/linux/page_reporting.h | 26 ++
include/uapi/linux/virtio_balloon.h | 1
mm/Kconfig | 11 +
mm/Makefile | 1
mm/internal.h | 2
mm/page_alloc.c | 164 ++++++++++----
mm/page_isolation.c | 6
mm/page_reporting.c | 364 ++++++++++++++++++++++++++++++
mm/page_reporting.h | 54 ++++
mm/shuffle.c | 12 -
mm/shuffle.h | 6
16 files changed, 725 insertions(+), 106 deletions(-)
create mode 100644 Documentation/vm/free_page_reporting.rst
create mode 100644 include/linux/page_reporting.h
create mode 100644 mm/page_reporting.c
create mode 100644 mm/page_reporting.h

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