[Devel] [PATCH vz7] mm: check for SIGKILL inside dup_mmap() loop

[Alexander Atanasov]

As a theoretical problem, dup_mmap() of an mm_struct with 60000+ vmas
can loop while potentially allocating memory, with mm->mmap_sem held for
write by current thread.  This is bad if current thread was selected as
an OOM victim, for current thread will continue allocations using memory
reserves while OOM reaper is unable to reclaim memory.

As an actually observable problem, it is not difficult to make OOM
reaper unable to reclaim memory if the OOM victim is blocked at
i_mmap_lock_write() in this loop.  Unfortunately, since nobody can
explain whether it is safe to use killable wait there, let's check for
SIGKILL before trying to allocate memory.  Even without an OOM event,
there is no point with continuing the loop from the beginning if current
thread is killed.

I tested with debug printk().  This patch should be safe because we
already fail if security_vm_enough_memory_mm() or
kmem_cache_alloc(GFP_KERNEL) fails and exit_mmap() handles it.

   ***** Aborting dup_mmap() due to SIGKILL *****
   ***** Aborting dup_mmap() due to SIGKILL *****
   ***** Aborting dup_mmap() due to SIGKILL *****
   ***** Aborting dup_mmap() due to SIGKILL *****
   ***** Aborting exit_mmap() due to NULL mmap *****

 [akpm at linux-foundation.org: add comment]
 Link: http://lkml.kernel.org/r/201804071938.CDE04681.SOFVQJFtMHOOLF@I-love.SAKURA.ne.jp
 Signed-off-by: Tetsuo Handa <penguin-kernel at I-love.SAKURA.ne.jp>
 Cc: Alexander Viro <viro at zeniv.linux.org.uk>
 Cc: Rik van Riel <riel at redhat.com>
 Cc: Michal Hocko <mhocko at suse.com>
 Cc: Kirill A. Shutemov <kirill.shutemov at linux.intel.com>
 Cc: David Rientjes <rientjes at google.com>
 Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
 Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>

(cherry picked from commit 655c79bb40a0870adcd0871057d01de11625882b)
https://jira.sw.ru/browse/PSBM-143415
Signed-off-by: Alexander Atanasov <alexander.atanasov at virtuozzo.com>
---
 kernel/fork.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/fork.c b/kernel/fork.c
index 75ae37b0bf7b..9bb8efdad181 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -483,6 +483,14 @@ static int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm)
 			continue;
 		}
 		charge = 0;
+		/*
+		 * Don't duplicate many vmas if we've been oom-killed (for
+		 * example)
+		 */
+		if (fatal_signal_pending(current)) {
+			retval = -EINTR;
+			goto out;
+		}
 		if (ub_memory_charge(mm, mpnt->vm_end - mpnt->vm_start,
 					mpnt->vm_flags & ~VM_LOCKED,
 					mpnt->vm_file, UB_HARD))
-- 
2.31.1