[PATCH mm-unstable 1/1] fs/proc/task_mmu: advance seq_file position on interrupted VMA lookup

["Adrian Huang (Lenovo)" <adrianhuang0701@gmail.com>]

Commit 5631da56c9a8 ("fs/proc/task_mmu: read proc/pid/maps under
per-vma lock") introduced the possibility of returning -EINTR
from proc_get_vma() when the VMA lookup is interrupted.

When this happens in m_next(), proc_get_vma() returns an error
pointer without updating the position index. Consequently,
seq_read_iter() reports:

  seq_file: buggy .next function m_next did not update position index

The above buggy message is reproduced by the following command:

  # stress-ng --vma 0 -t 60

Update the position index when VMA lookup fails, as required by
the seq_file interface. Also save the original position in
priv->last_pos so that subsequent iteration can restart from
the correct address without skipping or repeating a VMA.

Fixes: 5631da56c9a8 ("fs/proc/task_mmu: read proc/pid/maps under per-vma lock")
Signed-off-by: Adrian Huang (Lenovo) <adrianhuang0701@gmail.com>
---
 fs/proc/task_mmu.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 22361607020d..164b4efc5e09 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -221,6 +221,16 @@ static struct vm_area_struct *proc_get_vma(struct seq_file *m, loff_t *ppos)
 		if (PTR_ERR(vma) == -EAGAIN && fallback_to_mmap_lock(priv, *ppos))
 			goto retry;
 
+		/*
+		 * seq_file checks that the position advanced before it looks at
+		 * the returned pointer, so an error return that leaves *ppos
+		 * alone is reported as a buggy ->next. Advance it, and keep
+		 * last_pos at the pending position so m_start() would resume
+		 * the lookup from there.
+		 */
+		priv->last_pos = *ppos;
+		(*ppos)++;
+
 		return vma;
 	}
 
-- 
2.43.0