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Message-ID: <20251006181835.1919496-16-sashal@kernel.org>
Date: Mon, 6 Oct 2025 14:17:48 -0400
From: Sasha Levin <sashal@...nel.org>
To: patches@...ts.linux.dev,
stable@...r.kernel.org
Cc: Steven Rostedt <rostedt@...dmis.org>,
"Peter Zijlstra (Intel)" <peterz@...radead.org>,
Sasha Levin <sashal@...nel.org>,
mingo@...hat.com,
acme@...nel.org,
namhyung@...nel.org,
linux-perf-users@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: [PATCH AUTOSEL 6.17-6.6] perf: Use current->flags & PF_KTHREAD|PF_USER_WORKER instead of current->mm == NULL
From: Steven Rostedt <rostedt@...dmis.org>
[ Upstream commit 90942f9fac05702065ff82ed0bade0d08168d4ea ]
To determine if a task is a kernel thread or not, it is more reliable to
use (current->flags & (PF_KTHREAD|PF_USER_WORKERi)) than to rely on
current->mm being NULL. That is because some kernel tasks (io_uring
helpers) may have a mm field.
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lore.kernel.org/r/20250820180428.592367294@kernel.org
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
LLM Generated explanations, may be completely bogus:
Based on my comprehensive analysis, here is my determination:
## **Backport Status: YES**
### Extensive Analysis:
#### **Context and Problem Being Fixed:**
This commit addresses a **correctness bug** in how the perf subsystem
identifies kernel threads. The issue is that the code was using
`current->mm == NULL` to determine if a task is a kernel thread, but
this assumption is incorrect for **io_uring helper threads and other
PF_USER_WORKER threads**, which have a non-NULL mm field despite being
kernel-side workers.
#### **Evidence from Code Investigation:**
1. **This is part of a fix series**: I found three related commits in
upstream:
- `16ed389227651`: "perf: Skip user unwind if the task is a kernel
thread" (already being backported to stable as `823d7b9ec8616`)
- `d77e3319e3109`: "perf: Simplify get_perf_callchain() user logic"
(already in stable as `96681d3b99282`)
- `90942f9fac057`: **This commit** - completes the fix by updating
remaining locations
2. **Historical context**: PF_USER_WORKER was introduced in commit
`54e6842d0775b` (March 2023) to handle io_uring and vhost workers
that behave differently from regular kernel threads. These threads
have mm contexts but shouldn't be treated as user threads for
operations like register sampling.
3. **Real-world impact**: PowerPC already experienced crashes (commit
`01849382373b8`) when trying to access pt_regs for PF_IO_WORKER tasks
during coredump generation, demonstrating this class of bugs is real.
#### **Specific Code Changes Analysis:**
1. **kernel/events/callchain.c:247-250** (currently at line 245 in
autosel-6.17):
- **OLD**: `if (current->mm)` then use `task_pt_regs(current)`
- **NEW**: `if (current->flags & (PF_KTHREAD | PF_USER_WORKER))` then
skip user unwinding
- **Impact**: Prevents perf from attempting to unwind user stack for
io_uring helpers
2. **kernel/events/core.c:7455** (currently at line 7443 in
autosel-6.17):
- **OLD**: `!(current->flags & PF_KTHREAD)`
- **NEW**: `!(current->flags & (PF_KTHREAD | PF_USER_WORKER))`
- **Impact**: Correctly excludes user worker threads from user
register sampling
3. **kernel/events/core.c:8095** (currently at line 8083 in
autosel-6.17):
- **OLD**: `if (current->mm != NULL)`
- **NEW**: `if (!(current->flags & (PF_KTHREAD | PF_USER_WORKER)))`
- **Impact**: Prevents incorrect page table walks for user worker
threads in `perf_virt_to_phys()`
#### **Why This Qualifies for Backporting:**
1. **Fixes a real bug**: Perf incorrectly handles io_uring helper
threads, leading to:
- Incorrect callchain collection
- Wrong register samples
- Potential crashes or data corruption when walking page tables
2. **Affects widely-used functionality**: io_uring is heavily used in
modern applications (databases, proxies, async I/O workloads), and
perf profiling of these workloads would hit this bug
3. **Small and contained**: Only 3 conditional checks changed across 2
files - minimal risk
4. **Part of an upstream series already being backported**: The first
commit in the series (`16ed389227651`) is already marked for stable
backport, making this a natural follow-up
5. **No architectural changes**: Pure bug fix with no API changes or
feature additions
6. **Low regression risk**: The new flag-based check is more correct
than the mm-based check; any behavioral changes would be fixing
incorrect behavior
#### **Verification in Current Tree:**
I confirmed that linux-autosel-6.17 still has the old buggy code:
- Line 7443: Missing PF_USER_WORKER in the check
- Line 8083: Still uses `current->mm != NULL`
- callchain.c:245: Still uses `if (current->mm)`
This confirms the fix is needed and not yet applied.
#### **Conclusion:**
This is a **clear YES for backporting**. It's a well-understood
correctness fix for a real bug affecting perf profiling of io_uring
workloads, with minimal risk and part of an ongoing stable backport
series.
kernel/events/callchain.c | 6 +++---
kernel/events/core.c | 4 ++--
2 files changed, 5 insertions(+), 5 deletions(-)
diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
index 6c83ad674d010..decff7266cfbd 100644
--- a/kernel/events/callchain.c
+++ b/kernel/events/callchain.c
@@ -242,10 +242,10 @@ get_perf_callchain(struct pt_regs *regs, u32 init_nr, bool kernel, bool user,
if (user) {
if (!user_mode(regs)) {
- if (current->mm)
- regs = task_pt_regs(current);
- else
+ if (current->flags & (PF_KTHREAD | PF_USER_WORKER))
regs = NULL;
+ else
+ regs = task_pt_regs(current);
}
if (regs) {
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 820127536e62b..ea9ff856770be 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7440,7 +7440,7 @@ static void perf_sample_regs_user(struct perf_regs *regs_user,
if (user_mode(regs)) {
regs_user->abi = perf_reg_abi(current);
regs_user->regs = regs;
- } else if (!(current->flags & PF_KTHREAD)) {
+ } else if (!(current->flags & (PF_KTHREAD | PF_USER_WORKER))) {
perf_get_regs_user(regs_user, regs);
} else {
regs_user->abi = PERF_SAMPLE_REGS_ABI_NONE;
@@ -8080,7 +8080,7 @@ static u64 perf_virt_to_phys(u64 virt)
* Try IRQ-safe get_user_page_fast_only first.
* If failed, leave phys_addr as 0.
*/
- if (current->mm != NULL) {
+ if (!(current->flags & (PF_KTHREAD | PF_USER_WORKER))) {
struct page *p;
pagefault_disable();
--
2.51.0
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