[PATCH v3] mm: memory-failure: use rcu lock instead of tasklist_lock when collect_procs()

From: Tong Tiangen
Date: Sun Aug 27 2023 - 22:29:21 EST

We found a softlock issue in our test, analyzed the logs, and found that
the relevant CPU call trace as follows:

CPU0:
_do_fork
-> copy_process()
-> write_lock_irq(&tasklist_lock) //Disable irq,waiting for
//tasklist_lock

CPU1:
wp_page_copy()
->pte_offset_map_lock()
-> spin_lock(&page->ptl); //Hold page->ptl
-> ptep_clear_flush()
-> flush_tlb_others() ...
-> smp_call_function_many()
-> arch_send_call_function_ipi_mask()
-> csd_lock_wait() //Waiting for other CPUs respond
//IPI

CPU2:
collect_procs_anon()
-> read_lock(&tasklist_lock) //Hold tasklist_lock
->for_each_process(tsk)
-> page_mapped_in_vma()
-> page_vma_mapped_walk()
-> map_pte()
->spin_lock(&page->ptl) //Waiting for page->ptl

We can see that CPU1 waiting for CPU0 respond IPI，CPU0 waiting for CPU2
unlock tasklist_lock, CPU2 waiting for CPU1 unlock page->ptl. As a result,
softlockup is triggered.

For collect_procs_anon(), what we're doing is task list iteration, during
the iteration, with the help of call_rcu(), the task_struct object is freed
only after one or more grace periods elapse. the logic as follows:

release_task()
-> __exit_signal()
-> __unhash_process()
-> list_del_rcu()

-> put_task_struct_rcu_user()
-> call_rcu(&task->rcu, delayed_put_task_struct)

delayed_put_task_struct()
-> put_task_struct()
-> if (refcount_sub_and_test())
__put_task_struct()
-> free_task()

Therefore, under the protection of the rcu lock, we can safely use
get_task_struct() to ensure a safe reference to task_struct during the
iteration.

By removing the use of tasklist_lock in task list iteration, we can break
the softlock chain above.

The same logic can also be applied to:
- collect_procs_file()
- collect_procs_fsdax()
- collect_procs_ksm()

Signed-off-by: Tong Tiangen <tongtiangen@xxxxxxxxxx>
Acked-by: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
---
Since v2:
- 1. According to the analysis of Naoya，Matthew and Kefeng，update
the commit message.

Since v1:
- 1. According to Matthew's suggestion, only the comments of
find_early_kill_thread() are modified, no need to hold the rcu lock.

Changes since RFC[1]:
- 1. According to Naoya's suggestion, modify the tasklist_lock in the
comment about locking order in mm/filemap.c.
- 2. According to Kefeng's suggestion, optimize the implementation of
find_early_kill_thread() without functional changes.
- 3. Modify the title description.

[1] https://lore.kernel.org/lkml/20230815130154.1100779-1-tongtiangen@xxxxxxxxxx/
---
mm/filemap.c | 3 ---
mm/ksm.c | 4 ++--
mm/memory-failure.c | 16 ++++++++--------
3 files changed, 10 insertions(+), 13 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 014b73eb96a1..dfade1ef1765 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -121,9 +121,6 @@
* bdi.wb->list_lock (zap_pte_range->set_page_dirty)
* ->inode->i_lock (zap_pte_range->set_page_dirty)
* ->private_lock (zap_pte_range->block_dirty_folio)
- *
- * ->i_mmap_rwsem
- * ->tasklist_lock (memory_failure, collect_procs_ao)
*/

static void page_cache_delete(struct address_space *mapping,
diff --git a/mm/ksm.c b/mm/ksm.c
index 8d6aee05421d..981af9c72e7a 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -2925,7 +2925,7 @@ void collect_procs_ksm(struct page *page, struct list_head *to_kill,
struct anon_vma *av = rmap_item->anon_vma;

anon_vma_lock_read(av);
- read_lock(&tasklist_lock);
+ rcu_read_lock();
for_each_process(tsk) {
struct anon_vma_chain *vmac;
unsigned long addr;
@@ -2944,7 +2944,7 @@ void collect_procs_ksm(struct page *page, struct list_head *to_kill,
}
}
}
- read_unlock(&tasklist_lock);
+ rcu_read_unlock();
anon_vma_unlock_read(av);
}
}
diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 7b01fffe7a79..4d6e43c88489 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -547,8 +547,8 @@ static void kill_procs(struct list_head *to_kill, int forcekill, bool fail,
* on behalf of the thread group. Return task_struct of the (first found)
* dedicated thread if found, and return NULL otherwise.
*
- * We already hold read_lock(&tasklist_lock) in the caller, so we don't
- * have to call rcu_read_lock/unlock() in this function.
+ * We already hold rcu lock in the caller, so we don't have to call
+ * rcu_read_lock/unlock() in this function.
*/
static struct task_struct *find_early_kill_thread(struct task_struct *tsk)
{
@@ -609,7 +609,7 @@ static void collect_procs_anon(struct page *page, struct list_head *to_kill,
return;

pgoff = page_to_pgoff(page);
- read_lock(&tasklist_lock);
+ rcu_read_lock();
for_each_process(tsk) {
struct anon_vma_chain *vmac;
struct task_struct *t = task_early_kill(tsk, force_early);
@@ -626,7 +626,7 @@ static void collect_procs_anon(struct page *page, struct list_head *to_kill,
add_to_kill_anon_file(t, page, vma, to_kill);
}
}
- read_unlock(&tasklist_lock);
+ rcu_read_unlock();
anon_vma_unlock_read(av);
}

@@ -642,7 +642,7 @@ static void collect_procs_file(struct page *page, struct list_head *to_kill,
pgoff_t pgoff;

i_mmap_lock_read(mapping);
- read_lock(&tasklist_lock);
+ rcu_read_lock();
pgoff = page_to_pgoff(page);
for_each_process(tsk) {
struct task_struct *t = task_early_kill(tsk, force_early);
@@ -662,7 +662,7 @@ static void collect_procs_file(struct page *page, struct list_head *to_kill,
add_to_kill_anon_file(t, page, vma, to_kill);
}
}
- read_unlock(&tasklist_lock);
+ rcu_read_unlock();
i_mmap_unlock_read(mapping);
}

@@ -685,7 +685,7 @@ static void collect_procs_fsdax(struct page *page,
struct task_struct *tsk;

i_mmap_lock_read(mapping);
- read_lock(&tasklist_lock);
+ rcu_read_lock();
for_each_process(tsk) {
struct task_struct *t = task_early_kill(tsk, true);

@@ -696,7 +696,7 @@ static void collect_procs_fsdax(struct page *page,
add_to_kill_fsdax(t, page, vma, to_kill, pgoff);
}
}
- read_unlock(&tasklist_lock);
+ rcu_read_unlock();
i_mmap_unlock_read(mapping);
}
#endif /* CONFIG_FS_DAX */
--
2.25.1