Re: [PATCH 1/1] mm/vmalloc: Add preempt point in purge_vmap_node() when enabling kasan

Adrian Huang
2024-07-22T11:50:59Z

> I tried to simulate the reported splat and i can reproduce it with KASAN
> enabled. I use qemu on my 64-core system, it allows me to specify 255
> cores while running VM. The kernel is 6.10.0-rc5.
>
> The kernel should be built with CONFIG_KASAN=y and CONFIG_KASAN_VMALLOC=y
>
> The "soft lockup" can be triggered when the kernel is compiled within a
> VM using 256 jobs and preemption is disabled:
>
> echo none > /sys/kernel/debug/sched/preempt
> make -C coding/linux.git/ -j256 bzImage
>
> <snip>
> watchdog: BUG: soft lockup - CPU#28 stuck for 22s! [kworker/28:1:1760]
> CPU: 28 PID: 1760 Comm: kworker/28:1 Kdump: loaded Not tainted 6.10.0-rc5 #95
> Workqueue: events drain_vmap_area_work
> RIP: 0010:smp_call_function_many_cond+0x1d8/0xbb0
> ...
> <snip>

Great to hear you're able to reproduce the issue.

I keep debugging, and the original patch (https://lore.kernel.org/all/ZogS_04dP5LlRlXN@pc636/T/) shows purge_vmap_node() iteratively releases kasan vmalloc
allocations and flushes tlb for each vmap_area. There are 2805
flush_tlb_kernel_range() calls in ftrace log.
* One is called in __purge_vmap_area_lazy().
* Others are called in kasan_release_vmalloc(): Called by purge_vmap_node().
- [Rough calculation] Each flush_tlb_kernel_range() runs about 7.5ms.
-- 2804 * 7.5ms = 21.03 seconds (That's why a soft lock is trigger)

If we combine all tlb flush operations into one operation in the call path
'purge_vmap_node()->kasan_release_vmalloc()', the running time of
drain_vmap_area_work() can be saved greately. The idea is from the
flush_tlb_kernel_range() call in __purge_vmap_area_lazy().
And, the soft lockup won't not be triggered. Please refer to the following patch.
Here is the test result based on 6.10:

[6.10 wo/ the patch below]
1. ftrace latency profiling (record a trace if the latency > 20s): Commands
echo 20000000 > /sys/kernel/debug/tracing/tracing_thresh
echo drain_vmap_area_work > /sys/kernel/debug/tracing/set_graph_function
echo function_graph > /sys/kernel/debug/tracing/current_tracer
echo 1 > /sys/kernel/debug/tracing/tracing_on

2. Run `make -j $(nproc)` to compile the kernel source

3. Once the soft lockup is reproduced, check the ftace:
cat /sys/kernel/debug/tracing/trace
# tracer: function_graph
#
# CPU DURATION FUNCTION CALLS
# | | | | | | |
76) $ 50412985 us | } /* __purge_vmap_area_lazy */
76) $ 50412997 us | } /* drain_vmap_area_work */
76) $ 29165911 us | } /* __purge_vmap_area_lazy */
76) $ 29165926 us | } /* drain_vmap_area_work */
91) $ 53629423 us | } /* __purge_vmap_area_lazy */
91) $ 53629434 us | } /* drain_vmap_area_work */
91) $ 28121014 us | } /* __purge_vmap_area_lazy */
91) $ 28121026 us | } /* drain_vmap_area_work */

[6.10 w/ the patch below]
1. Repeat step 1-2 in "[6.10 wo/ the patch below]"

2. The soft lockup is not triggered and the ftrace log is empty.
cat /sys/kernel/debug/tracing/trace
# tracer: function_graph
#
# CPU DURATION FUNCTION CALLS
# | | | | | | |

3. Setting 'tracing_thresh' to 10/5 seconds does not get any ftrace log.

4. Setting 'tracing_thresh' to 1 second gets ftrace log.
cat /sys/kernel/tracing/trace
# tracer: function_graph
#
# CPU DURATION FUNCTION CALLS
# | | | | | | |
51) $ 1019695 us | } /* __purge_vmap_area_lazy */
51) $ 1019703 us | } /* drain_vmap_area_work */
198) $ 1018707 us | } /* __purge_vmap_area_lazy */
198) $ 1018718 us | } /* drain_vmap_area_work */

5. Run the following test_vmalloc command without any issues
modprobe test_vmalloc nr_threads=$(nproc) run_test_mask=0x1 nr_pages=4

Could you please test this patch in your VM environment?

---
diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index 70d6a8f6e25d..ddbf42a1a7b7 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -55,6 +55,9 @@ extern p4d_t kasan_early_shadow_p4d[MAX_PTRS_PER_P4D];
int kasan_populate_early_shadow(const void *shadow_start,
const void *shadow_end);

+#define KASAN_VMALLOC_PAGE_RANGE 0x1 /* Apply existing page range */
+#define KASAN_VMALLOC_TLB_FLUSH 0x2 /* TLB flush */
+
#ifndef kasan_mem_to_shadow
static inline void *kasan_mem_to_shadow(const void *addr)
{
@@ -511,7 +514,8 @@ void kasan_populate_early_vm_area_shadow(void *start, unsigned long size);
int kasan_populate_vmalloc(unsigned long addr, unsigned long size);
void kasan_release_vmalloc(unsigned long start, unsigned long end,
unsigned long free_region_start,
- unsigned long free_region_end);
+ unsigned long free_region_end,
+ unsigned long flags);

#else /* CONFIG_KASAN_GENERIC || CONFIG_KASAN_SW_TAGS */

@@ -526,7 +530,8 @@ static inline int kasan_populate_vmalloc(unsigned long start,
static inline void kasan_release_vmalloc(unsigned long start,
unsigned long end,
unsigned long free_region_start,
- unsigned long free_region_end) { }
+ unsigned long free_region_end,
+ unsigned long flags) { }

#endif /* CONFIG_KASAN_GENERIC || CONFIG_KASAN_SW_TAGS */

@@ -561,7 +566,8 @@ static inline int kasan_populate_vmalloc(unsigned long start,
static inline void kasan_release_vmalloc(unsigned long start,
unsigned long end,
unsigned long free_region_start,
- unsigned long free_region_end) { }
+ unsigned long free_region_end,
+ unsigned long flags) { }

static inline void *kasan_unpoison_vmalloc(const void *start,
unsigned long size,
diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
index d6210ca48dda..88d1c9dcb507 100644
--- a/mm/kasan/shadow.c
+++ b/mm/kasan/shadow.c
@@ -489,7 +489,8 @@ static int kasan_depopulate_vmalloc_pte(pte_t *ptep, unsigned long addr,
*/
void kasan_release_vmalloc(unsigned long start, unsigned long end,
unsigned long free_region_start,
- unsigned long free_region_end)
+ unsigned long free_region_end,
+ unsigned long flags)
{
void *shadow_start, *shadow_end;
unsigned long region_start, region_end;
@@ -522,12 +523,17 @@ void kasan_release_vmalloc(unsigned long start, unsigned long end,
__memset(shadow_start, KASAN_SHADOW_INIT, shadow_end - shadow_start);
return;
}
- apply_to_existing_page_range(&init_mm,
+
+
+ if (flags & KASAN_VMALLOC_PAGE_RANGE)
+ apply_to_existing_page_range(&init_mm,
(unsigned long)shadow_start,
size, kasan_depopulate_vmalloc_pte,
NULL);
- flush_tlb_kernel_range((unsigned long)shadow_start,
- (unsigned long)shadow_end);
+
+ if (flags & KASAN_VMALLOC_TLB_FLUSH)
+ flush_tlb_kernel_range((unsigned long)shadow_start,
+ (unsigned long)shadow_end);
}
}

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index e34ea860153f..d66e09135876 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2193,8 +2193,15 @@ static void purge_vmap_node(struct work_struct *work)
struct vmap_area *va, *n_va;
LIST_HEAD(local_list);

+ unsigned long start;
+ unsigned long end;
+
vn->nr_purged = 0;

+ start = list_first_entry(&vn->purge_list, struct vmap_area, list)->va_start;
+
+ end = list_last_entry(&vn->purge_list, struct vmap_area, list)->va_end;
+
list_for_each_entry_safe(va, n_va, &vn->purge_list, list) {
unsigned long nr = (va->va_end - va->va_start) >> PAGE_SHIFT;
unsigned long orig_start = va->va_start;
@@ -2205,7 +2212,8 @@ static void purge_vmap_node(struct work_struct *work)

if (is_vmalloc_or_module_addr((void *)orig_start))
kasan_release_vmalloc(orig_start, orig_end,
- va->va_start, va->va_end);
+ va->va_start, va->va_end,
+ KASAN_VMALLOC_PAGE_RANGE);

atomic_long_sub(nr, &vmap_lazy_nr);
vn->nr_purged++;
@@ -2218,6 +2226,8 @@ static void purge_vmap_node(struct work_struct *work)
list_add(&va->list, &local_list);
}

+ kasan_release_vmalloc(start, end, start, end, KASAN_VMALLOC_TLB_FLUSH);
+
reclaim_list_global(&local_list);
}

@@ -4726,7 +4736,8 @@ struct vm_struct **pcpu_get_vm_areas(const unsigned long *offsets,
&free_vmap_area_list);
if (va)
kasan_release_vmalloc(orig_start, orig_end,
- va->va_start, va->va_end);
+ va->va_start, va->va_end,
+ KASAN_VMALLOC_PAGE_RANGE | KASAN_VMALLOC_TLB_FLUSH);
vas[area] = NULL;
}

@@ -4776,7 +4787,8 @@ struct vm_struct **pcpu_get_vm_areas(const unsigned long *offsets,
&free_vmap_area_list);
if (va)
kasan_release_vmalloc(orig_start, orig_end,
- va->va_start, va->va_end);
+ va->va_start, va->va_end,
+ KASAN_VMALLOC_PAGE_RANGE | KASAN_VMALLOC_TLB_FLUSH);
vas[area] = NULL;
kfree(vms[area]);
}