Summary ======= This patch series adds the ability to configure the kernel stack size for arm64 from the kernel command line.
On some systems (specifically, Android), this provides a mechanism to reduce the kernel memory consumption, as the default kernel stack size of 16kB (with 4kB pages) can result in ~100MB of allocated memory, despite the fact that most threads do not come close to exhausting their allocation [1]. There have been multiple alternative attempts to improve this situation, including for x86 and cloud workloads. However, these have typically focussed on more dynamic behaviours such as allocating kernel stack pages lazily (based on faults) [2] or reclaiming unused stack pages from blocked tasks [1], whereas this series focusses on making the kernel stack size configurable without changing the way in which it is allocated. It also permits a 12kB stack size, which is currently not supported by arm64's stack overflow checking logic. This series re-uses the first part of the dynamic stack patches which makes it possible to partially back the VA space of the stack (THREAD_SIZE) with memory, but never attempts to dynamically grow the stack. The size of the stack (<= THREAD_SIZE) is determined from the kernel command line and is not further configurable at runtime, all threads on the system (except the init_task, see below) will use the size set from the command line. Patches ======= The patches have dependency on the ongoing arm re-work[3] to move the overflow stack to sp_el1 allowing the early kernel exception to use it without clobbering any registers. - Patches 01-09: have no functional change, they abstract the code dealing with the stack to avoid hardcoding the stack size - Patches 10-13: Introduce the new ARCH_HAS_VARIABLE_STACK_SIZE and make the kernel deal with stack sizes set in the run time. - Patches 14-15: arm64 selecting ARCH_HAS_VARIABLE_STACK_SIZE and setting the kernel stack size from the command line. KASAN ====== The KASAN stack helpers keep unpoisoning the whole THREAD_SIZE area. As they only write shadow memory which is populated for the whole vmap area. init_task ========= init_task is the only kernel thread that has a full stack allocation as it is allocated statically. However, the additional stack pages are not usable because the overflow check on exception entry will continue to check against the configured stack size. Future work =========== There are multiple paths that can build on this - Per task stack size (either via an in-kernel API for kthreads or potentially a prctl() for userspace to configure) - It is still possible to build on top of this on the fault path to add dynamic stacks if the challenges raised in [2] can be solved. Will Deacon will host a discussion at LPC next week [4] Testing ======= I tested on Lenovo Mini-x gen 10 (Qualcomm X1 CPU). With configs KMEMLEAK, DEBUG_STACK_USAGE, SCHED_STACK_END_CHECK. 1) 4kB kernel - 16kB stack (default) 2) 4kB kernel - 8kB stack 3) 4kB kernel - 12kB stack 4) 64kB kernel - 64kB stack With running VMs with KVM, stress-ng and LKDTM. 16kB tests done on Qemu. [1] https://lore.kernel.org/all/[email protected]/ [2] https://lore.kernel.org/all/[email protected]/ [3] https://lore.kernel.org/all/[email protected]/ [4] https://lpc.events/event/20/contributions/2419/ David Stevens (3): fork: Don't assume fully populated stack during reuse fork: Move vm_stack to the beginning of the stack fork: Move vmap stack freeing to work queue Mostafa Saleh (9): sched/task_stack: Add helpers for stack high/low exit: Don't assume the kernel stack size usercopy: Don't assume the kernel stack size mm: kmemleak: Don't assume the kernel stack size arm64: Don't assume the kernel stack size sched/task_stack: Introduce ARCH_HAS_VARIABLE_STACK_SIZE fork: Implement partial VMAP stack allocation arm64: mm: Relax kernel stack alignment arm64: mm: Set stack size from the kernel command line Pasha Tatashin (3): fork: Remove assumption that vm_area->nr_pages equals to THREAD_SIZE fork: Separate vmap stack allocation and free calls mm/vmalloc: Add a get_vm_area_node() .../admin-guide/kernel-parameters.txt | 7 + arch/Kconfig | 10 ++ arch/arm64/Kconfig | 1 + arch/arm64/include/asm/memory.h | 19 ++- arch/arm64/include/asm/stacktrace.h | 4 +- arch/arm64/kernel/entry.S | 91 ++++++++++-- arch/arm64/kernel/setup.c | 24 ++++ arch/arm64/kernel/traps.c | 5 +- include/linux/sched/task_stack.h | 61 +++++++- include/linux/thread_info.h | 8 ++ include/linux/vmalloc.h | 3 + kernel/exit.c | 2 +- kernel/fork.c | 134 +++++++++++++++--- mm/kmemleak.c | 2 +- mm/usercopy.c | 4 +- mm/vmalloc.c | 24 ++++ 16 files changed, 348 insertions(+), 51 deletions(-) -- 2.56.0.rc1.315.gc6ed9934b7-goog
