Re: [RFC v1 0/2] Memory poison recovery in khugepaged

[Jiaqi Yan <jiaqiyan@google.com>]

On Thu, Mar 24, 2022 at 7:51 PM Yang Shi <shy828301@gmail.com> wrote:
>
> On Wed, Mar 23, 2022 at 4:29 PM Jiaqi Yan <jiaqiyan@google.com> wrote:
> >
> > 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.
> >
> > Impact
> > ======
> > The main reason we chose to make khugepaged 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.
> >
> > 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.
>
> Actually I should asked the question in the first place before diving
> into the implementation details, if uncorrectable memory error
> happens, kernel will pin the poisoned page and set hwpoison flag, the
> bumped page refcount would prevent the page from being collapsed IIUC.

This patch series is for cases where khugepaged is the first guy that detects
the memory errors on these 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 cloud VMs.



> So I'm wondering why we need this?
>
> >
> > Jiaqi Yan (2):
> >   mm: khugepaged: recover from poisoned anonymous memory
> >   mm: khugepaged: recover from poisoned file-backed memory
> >
> >  include/linux/highmem.h |  37 +++++++
> >  mm/khugepaged.c         | 211 +++++++++++++++++++++++++++++-----------
> >  2 files changed, 189 insertions(+), 59 deletions(-)
> >
> > --
> > 2.35.1.894.gb6a874cedc-goog
> >