Despite all the discussion, we're short on formal review/ack tags on this one. Here's what I have: From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> Subject: mm: check for SIGKILL inside dup_mmap() loop 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. [ 417.030691] ***** Aborting dup_mmap() due to SIGKILL ***** [ 417.036129] ***** Aborting dup_mmap() due to SIGKILL ***** [ 417.044544] ***** Aborting dup_mmap() due to SIGKILL ***** [ 419.116445] ***** Aborting dup_mmap() due to SIGKILL ***** [ 419.118401] ***** Aborting exit_mmap() due to NULL mmap ***** [akpm@xxxxxxxxxxxxxxxxxxxx: add comment] Link: http://lkml.kernel.org/r/201804071938.CDE04681.SOFVQJFtMHOOLF@xxxxxxxxxxxxxxxxxxx Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx> Cc: Rik van Riel <riel@xxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxx> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> Cc: David Rientjes <rientjes@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- kernel/fork.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff -puN kernel/fork.c~mm-check-for-sigkill-inside-dup_mmap-loop kernel/fork.c --- a/kernel/fork.c~mm-check-for-sigkill-inside-dup_mmap-loop +++ a/kernel/fork.c @@ -440,6 +440,14 @@ static __latent_entropy int dup_mmap(str 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 (mpnt->vm_flags & VM_ACCOUNT) { unsigned long len = vma_pages(mpnt); _
