[RFC 0/6] Reclaim zero subpages of thp to avoid memory bloat

[Ning Zhang <ningzhang@linux.alibaba.com>]

As we know, thp may lead to memory bloat which may cause OOM.
Through testing with some apps, we found that the reason of
memory bloat is a huge page may contain some zero subpages
(may accessed or not). And we found that most zero subpages
are centralized in a few huge pages.

Following is a text_classification_rnn case for tensorflow:

  zero_subpages   huge_pages  waste
  [     0,     1) 186         0.00%
  [     1,     2) 23          0.01%
  [     2,     4) 36          0.02%
  [     4,     8) 67          0.08%
  [     8,    16) 80          0.23%
  [    16,    32) 109         0.61%
  [    32,    64) 44          0.49%
  [    64,   128) 12          0.30%
  [   128,   256) 28          1.54%
  [   256,   513) 159        18.03%

In the case, there are 187 huge pages (25% of the total huge pages)
which contain more then 128 zero subpages. And these huge pages
lead to 19.57% waste of the total rss. It means we can reclaim
19.57% memory by splitting the 187 huge pages and reclaiming the
zero subpages.

This patchset introduce a new mechanism to split the huge page
which has zero subpages and reclaim these zero subpages.

We add the anonymous huge page to a list to reduce the cost of
finding the huge page. When the memory reclaim is triggering,
the list will be walked and the huge page contains enough zero
subpages may be reclaimed. Meanwhile, replace the zero subpages
by ZERO_PAGE(0). 

Yu Zhao has done some similar work when the huge page is swap out
or migrated to accelerate[1]. While we do this in the normal memory
shrink path for the swapoff scene to avoid OOM.

In the future, we will do the proactive reclaim to reclaim the "cold"
huge page proactively. This is for keeping the performance of thp as
for as possible. In addition to that, some users want the memory usage
using thp is equal to the usage using 4K.

[1] https://lore.kernel.org/linux-mm/20210731063938.1391602-1-yuzhao@google.com/

Ning Zhang (6):
  mm, thp: introduce thp zero subpages reclaim
  mm, thp: add a global interface for zero subapges reclaim
  mm, thp: introduce zero subpages reclaim threshold
  mm, thp: introduce a controller to trigger zero subpages reclaim
  mm, thp: add some statistics for zero subpages reclaim
  mm, thp: add document for zero subpages reclaim

 Documentation/admin-guide/mm/transhuge.rst |  75 ++++++
 include/linux/huge_mm.h                    |  13 +
 include/linux/memcontrol.h                 |  26 ++
 include/linux/mm.h                         |   1 +
 include/linux/mm_types.h                   |   6 +
 include/linux/mmzone.h                     |   9 +
 mm/huge_memory.c                           | 374 ++++++++++++++++++++++++++++-
 mm/memcontrol.c                            | 243 +++++++++++++++++++
 mm/vmscan.c                                |  61 ++++-
 9 files changed, 805 insertions(+), 3 deletions(-)

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1.8.3.1