Sending out this RFC in part to guage community interest.
This patchset implements preserved-over-kexec memory storage or PKRAM as a
method for saving memory pages of the currently executing kernel so that
they may be restored after kexec into a new kernel. The patches are adapted
from an RFC patchset sent out in 2013 by Vladimir Davydov [1]. They
introduce the PKRAM kernel API.
One use case for PKRAM is preserving guest memory and/or auxillary
supporting data (e.g. iommu data) across kexec to support reboot of the
host with minimal disruption to the guest. PKRAM provides a flexible way
for doing this without requiring that the amount of memory used by a fixed
size created a priori. Another use case is for databases to preserve their
block caches in shared memory across reboot.
Changes since RFC v2
- Rebased onto 6.3
- Updated API to save/load folios rather than file pages
- Omitted previous patches for implementing and optimizing preservation
and restoration of shmem files to reduce the number of patches and
focus on core functionality.
Changes since RFC v1
- Rebased onto 5.12-rc4
- Refined the API to reduce the number of calls
and better support multithreading.
- Allow preserving byte data of arbitrary length
(was previously limited to one page).
- Build a new memblock reserved list with the
preserved ranges and then substitute it for
the existing one. (Mike Rapoport)
- Use mem_avoid_overlap() to avoid kaslr stepping
on preserved ranges. (Kees Cook)
-- Implementation details --
* To aid in quickly finding contiguous ranges of memory containing
preserved pages a pseudo physical mapping pagetable is populated
with pages as they are preserved.
* If a page to be preserved is found to be in range of memory that was
previously reserved during early boot or in range of memory where the
kernel will be loaded to on kexec, the page will be copied to a page
outside of those ranges and the new page will be preserved. A compound
page will be copied to and preserved as individual base pages.
Note that this means that a page that cannot be moved (e.g. pinned for
DMA) currently cannot safely be preserved. This could be addressed by
adding functionality to kexec to reconfigure the destination addreses
for the sections of an already-loaded kexec kernel.
* A single page is allocated for the PKRAM super block. For the next kernel
kexec boot to find preserved memory metadata, the pfn of the PKRAM super
block, which is exported via /sys/kernel/pkram, is passed in the 'pkram'
boot option.
* In the newly booted kernel, PKRAM adds all preserved pages to the memblock
reserve list during early boot so that they will not be recycled.
* Since kexec may load the new kernel code to any memory region, it could
destroy preserved memory. When the kernel selects the memory region
(kexec_file_load syscall), kexec will avoid preserved pages. When the
user selects the kexec memory region to use (kexec_load syscall) , kexec
load will fail if there is conflict with preserved pages. Pages preserved
after a kexec kernel is loaded will be relocated if they conflict with
the selected memory region.
[1] https://lkml.org/lkml/2013/7/1/211
Anthony Yznaga (21):
mm: add PKRAM API stubs and Kconfig
mm: PKRAM: implement node load and save functions
mm: PKRAM: implement object load and save functions
mm: PKRAM: implement folio stream operations
mm: PKRAM: implement byte stream operations
mm: PKRAM: link nodes by pfn before reboot
mm: PKRAM: introduce super block
PKRAM: track preserved pages in a physical mapping pagetable
PKRAM: pass a list of preserved ranges to the next kernel
PKRAM: prepare for adding preserved ranges to memblock reserved
mm: PKRAM: reserve preserved memory at boot
PKRAM: free the preserved ranges list
PKRAM: prevent inadvertent use of a stale superblock
PKRAM: provide a way to ban pages from use by PKRAM
kexec: PKRAM: prevent kexec clobbering preserved pages in some cases
PKRAM: provide a way to check if a memory range has preserved pages
kexec: PKRAM: avoid clobbering already preserved pages
mm: PKRAM: allow preserved memory to be freed from userspace
PKRAM: disable feature when running the kdump kernel
x86/KASLR: PKRAM: support physical kaslr
x86/boot/compressed/64: use 1GB pages for mappings
arch/x86/boot/compressed/Makefile | 3 +
arch/x86/boot/compressed/ident_map_64.c | 9 +-
arch/x86/boot/compressed/kaslr.c | 10 +-
arch/x86/boot/compressed/misc.h | 10 +
arch/x86/boot/compressed/pkram.c | 110 ++
arch/x86/kernel/setup.c | 3 +
arch/x86/mm/init_64.c | 3 +
include/linux/pkram.h | 116 ++
kernel/kexec.c | 9 +
kernel/kexec_core.c | 3 +
kernel/kexec_file.c | 15 +
mm/Kconfig | 9 +
mm/Makefile | 2 +
mm/pkram.c | 1753 +++++++++++++++++++++++++++++++
mm/pkram_pagetable.c | 375 +++++++
15 files changed, 2424 insertions(+), 6 deletions(-)
create mode 100644 arch/x86/boot/compressed/pkram.c
create mode 100644 include/linux/pkram.h
create mode 100644 mm/pkram.c
create mode 100644 mm/pkram_pagetable.c
--
1.9.4
