[RFC,v2,0/2] Memory poison recovery in khugepaged collapsing

Message

Jiaqi Yan April 5, 2022, 8:51 p.m. UTC

Problem
=======
Memory DIMMs are subject to multi-bit flips, i.e. memory errors.
As memory size and density increase, the chances of and number of
memory errors increase. The increasing size and density of server
RAM in the data center and cloud have shown increased uncorrectable
memory errors. There are already mechanisms in the kernel to recover
from uncorrectable memory errors. This series of patches provides
the recovery mechanism for the particular kernel agent khugepaged
when it collapses memory pages.

Impact
======
The main reason we chose to make khugepaged collapsing tolerant of
memory failures was its high possibility of accessing poisoned memory
while performing functionally optional compaction actions.
Standard applications typically don't have strict requirements on
the size of its pages. So they are given 4K pages by the kernel.
The kernel is able to improve application performance by either

  1) giving application 2M pages to begin with, or
  2) collapsing 4K pages into 2M pages when possible.

This collapsing operation is done by khugepaged, a kernel agent that
is constantly scanning memory. When collapsing 4K pages into a 2M page,
it must copy the data from the 4K pages into a physically contiguous
2M page. Therefore, as long as there exists one poisoned cache line in
collapsible 4K pages, khugepaged will eventually access it. The current
impact to users is a machine check exception triggered kernel panic.
However, khugepaged’s compaction operations are not functionally required
kernel actions. Therefore making khugepaged tolerant to poisoned memory
will greatly improve user experience.

This patch series is for cases where khugepaged is the first guy
that detects the memory errors on the poisoned pages. IOW, the pages
are not known to have memory errors when khugepaged collapsing gets to
them. In our observation, this happens frequently when the huge page
ratio of the system is relatively low, which is fairly common in
virtual machines running on cloud.

Solution
========
As stated before, it is less desirable to crash the system only because
khugepaged accesses poisoned pages while it is collapsing 4K pages.
The high level idea of this patch series is to skip the group of pages
(usually 512 4K-size pages) once khugepaged finds one of them is poisoned,
as these pages have become ineligible to be collapsed.

We are also careful to unwind operations khuagepaged has performed before
it detects memory failures. For example, before copying and collapsing
a group of anonymous pages into a huge page, the source pages will be
isolated and their page table is unlinked from their PMD. These operations
need to be undone in order to ensure these pages are not changed/lost from
the perspective of other threads (both user and kernel space). As for
file backed memory pages, there already exists a rollback case. This
patch just extends it so that khugepaged also correctly rolls back when
it fails to copy poisoned 4K pages.

Jiaqi Yan (2):
  mm: khugepaged: recover from poisoned anonymous memory
  mm: khugepaged: recover from poisoned file-backed memory

 include/linux/highmem.h |  18 ++++
 mm/khugepaged.c         | 215 ++++++++++++++++++++++++++++------------
 2 files changed, 171 insertions(+), 62 deletions(-)