From: Simon Schuster schuster.simon@siemens-energy.com
[ Upstream commit a20b83cf45be2057f3d073506779e52c7fa17f94 ]
On nios2, with CONFIG_FLATMEM set, the kernel relies on memblock_get_current_limit() to determine the limits of mem_map, in particular for max_low_pfn. Unfortunately, memblock.current_limit is only default initialized to MEMBLOCK_ALLOC_ANYWHERE at this point of the bootup, potentially leading to situations where max_low_pfn can erroneously exceed the value of max_pfn and, thus, the valid range of available DRAM.
This can in turn cause kernel-level paging failures, e.g.:
[ 76.900000] Unable to handle kernel paging request at virtual address 20303000 [ 76.900000] ea = c0080890, ra = c000462c, cause = 14 [ 76.900000] Kernel panic - not syncing: Oops [ 76.900000] ---[ end Kernel panic - not syncing: Oops ]---
This patch fixes this by pre-calculating memblock.current_limit based on the upper limits of the available memory ranges via adjust_lowmem_bounds, a simplified version of the equivalent implementation within the arm architecture.
Signed-off-by: Simon Schuster schuster.simon@siemens-energy.com Signed-off-by: Andreas Oetken andreas.oetken@siemens-energy.com Signed-off-by: Dinh Nguyen dinguyen@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org ---
LLM Generated explanations, may be completely bogus:
Based on my comprehensive analysis, here is my determination:
## Backport Recommendation: **YES**
### Analysis
**Bug Description:** The commit fixes a critical memory initialization bug in nios2 architecture with CONFIG_FLATMEM. The `find_limits()` function (introduced in 2021 via commit 7f7bc20bc41a4) uses `memblock_get_current_limit()` to calculate `max_low_pfn` at arch/nios2/kernel/setup.c:140:
```c *max_low_pfn = PFN_DOWN(memblock_get_current_limit()); ```
However, `memblock.current_limit` is only default-initialized to `MEMBLOCK_ALLOC_ANYWHERE` (defined as `~(phys_addr_t)0` in include/linux/memblock.h:408, essentially ULONG_MAX). This causes `max_low_pfn` to be set to an enormous value that exceeds `max_pfn`, violating the critical invariant `max_low_pfn <= max_pfn`.
**Impact:** When the kernel attempts memory operations using the incorrect `max_low_pfn`, it accesses memory beyond actual DRAM, resulting in kernel paging failures: ``` Unable to handle kernel paging request at virtual address 20303000 Kernel panic - not syncing: Oops ```
**The Fix:** The commit adds `adjust_lowmem_bounds()` (arch/nios2/kernel/setup.c:145-157) which: 1. Iterates through all memory ranges using `for_each_mem_range()` 2. Finds the highest `block_end` address 3. Calls `memblock_set_current_limit(memblock_limit)` to set the actual memory limit
This function is called at line 174 before `find_limits()`, ensuring correct initialization. The implementation follows the proven pattern from ARM architecture (arch/arm/mm/mmu.c:1185).
**Backporting Justification:**
1. **Critical bug with clear user impact**: Causes kernel panics on affected systems 2. **Small and well-contained**: Only 15 lines of code added 3. **Minimal regression risk**: - Architecture-specific (nios2 only) - zero risk to other systems - Pattern proven in ARM for years (since commit 985626564eedc from 2017) - Only affects boot-time initialization 4. **Follows stable tree rules**: Important bugfix, no new features, no architectural changes 5. **Similar issues in other architectures**: MIPS had a similar `max_low_pfn` bug fixed in commit 0f5cc249ff735 (2023) 6. **Long-lived bug**: Latent since 2021, affects all kernels with commit 7f7bc20bc41a4
**Historical Context:** - Bug introduced: 2021-02-19 (commit 7f7bc20bc41a4) - Bug fixed: 2025-08-21 (commit a20b83cf45be2) - Same author (Andreas Oetken) involved in both commits
**Target Stable Trees:** Should be backported to all stable trees containing commit 7f7bc20bc41a4 (February 2021 onwards).
arch/nios2/kernel/setup.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+)
diff --git a/arch/nios2/kernel/setup.c b/arch/nios2/kernel/setup.c index 2a40150142c36..f43f01c4ab934 100644 --- a/arch/nios2/kernel/setup.c +++ b/arch/nios2/kernel/setup.c @@ -142,6 +142,20 @@ static void __init find_limits(unsigned long *min, unsigned long *max_low, *max_high = PFN_DOWN(memblock_end_of_DRAM()); }
+static void __init adjust_lowmem_bounds(void) +{ + phys_addr_t block_start, block_end; + u64 i; + phys_addr_t memblock_limit = 0; + + for_each_mem_range(i, &block_start, &block_end) { + if (block_end > memblock_limit) + memblock_limit = block_end; + } + + memblock_set_current_limit(memblock_limit); +} + void __init setup_arch(char **cmdline_p) { console_verbose(); @@ -157,6 +171,7 @@ void __init setup_arch(char **cmdline_p) /* Keep a copy of command line */ *cmdline_p = boot_command_line;
+ adjust_lowmem_bounds(); find_limits(&min_low_pfn, &max_low_pfn, &max_pfn);
memblock_reserve(__pa_symbol(_stext), _end - _stext);