From 212925802454672e6cd2949a727f5e2c1377bf06 Mon Sep 17 00:00:00 2001 From: Andrea Arcangeli Date: Wed, 6 Sep 2017 16:25:00 -0700 Subject: mm: oom: let oom_reap_task and exit_mmap run concurrently This is purely required because exit_aio() may block and exit_mmap() may never start, if the oom_reap_task cannot start running on a mm with mm_users == 0. At the same time if the OOM reaper doesn't wait at all for the memory of the current OOM candidate to be freed by exit_mmap->unmap_vmas, it would generate a spurious OOM kill. If it wasn't because of the exit_aio or similar blocking functions in the last mmput, it would be enough to change the oom_reap_task() in the case it finds mm_users == 0, to wait for a timeout or to wait for __mmput to set MMF_OOM_SKIP itself, but it's not just exit_mmap the problem here so the concurrency of exit_mmap and oom_reap_task is apparently warranted. It's a non standard runtime, exit_mmap() runs without mmap_sem, and oom_reap_task runs with the mmap_sem for reading as usual (kind of MADV_DONTNEED). The race between the two is solved with a combination of tsk_is_oom_victim() (serialized by task_lock) and MMF_OOM_SKIP (serialized by a dummy down_write/up_write cycle on the same lines of the ksm_exit method). If the oom_reap_task() may be running concurrently during exit_mmap, exit_mmap will wait it to finish in down_write (before taking down mm structures that would make the oom_reap_task fail with use after free). If exit_mmap comes first, oom_reap_task() will skip the mm if MMF_OOM_SKIP is already set and in turn all memory is already freed and furthermore the mm data structures may already have been taken down by free_pgtables. [aarcange@redhat.com: incremental one liner] Link: http://lkml.kernel.org/r/20170726164319.GC29716@redhat.com [rientjes@google.com: remove unused mmput_async] Link: http://lkml.kernel.org/r/alpine.DEB.2.10.1708141733130.50317@chino.kir.corp.google.com [aarcange@redhat.com: microoptimization] Link: http://lkml.kernel.org/r/20170817171240.GB5066@redhat.com Link: http://lkml.kernel.org/r/20170726162912.GA29716@redhat.com Fixes: 26db62f179d1 ("oom: keep mm of the killed task available") Signed-off-by: Andrea Arcangeli Signed-off-by: David Rientjes Reported-by: David Rientjes Tested-by: David Rientjes Reviewed-by: Michal Hocko Cc: Tetsuo Handa Cc: Oleg Nesterov Cc: Hugh Dickins Cc: "Kirill A. Shutemov" Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/mmap.c | 18 ++++++++++++++++++ mm/oom_kill.c | 15 +++++---------- 2 files changed, 23 insertions(+), 10 deletions(-) (limited to 'mm') diff --git a/mm/mmap.c b/mm/mmap.c index 52f6c6b18f40..4c5981651407 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -44,6 +44,7 @@ #include #include #include +#include #include #include @@ -3001,6 +3002,23 @@ void exit_mmap(struct mm_struct *mm) /* Use -1 here to ensure all VMAs in the mm are unmapped */ unmap_vmas(&tlb, vma, 0, -1); + set_bit(MMF_OOM_SKIP, &mm->flags); + if (unlikely(tsk_is_oom_victim(current))) { + /* + * Wait for oom_reap_task() to stop working on this + * mm. Because MMF_OOM_SKIP is already set before + * calling down_read(), oom_reap_task() will not run + * on this "mm" post up_write(). + * + * tsk_is_oom_victim() cannot be set from under us + * either because current->mm is already set to NULL + * under task_lock before calling mmput and oom_mm is + * set not NULL by the OOM killer only if current->mm + * is found not NULL while holding the task_lock. + */ + down_write(&mm->mmap_sem); + up_write(&mm->mmap_sem); + } free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING); tlb_finish_mmu(&tlb, 0, -1); diff --git a/mm/oom_kill.c b/mm/oom_kill.c index c9f3569a76c7..99736e026712 100644 --- a/mm/oom_kill.c +++ b/mm/oom_kill.c @@ -495,11 +495,12 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm) } /* - * increase mm_users only after we know we will reap something so - * that the mmput_async is called only when we have reaped something - * and delayed __mmput doesn't matter that much + * MMF_OOM_SKIP is set by exit_mmap when the OOM reaper can't + * work on the mm anymore. The check for MMF_OOM_SKIP must run + * under mmap_sem for reading because it serializes against the + * down_write();up_write() cycle in exit_mmap(). */ - if (!mmget_not_zero(mm)) { + if (test_bit(MMF_OOM_SKIP, &mm->flags)) { up_read(&mm->mmap_sem); trace_skip_task_reaping(tsk->pid); goto unlock_oom; @@ -542,12 +543,6 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm) K(get_mm_counter(mm, MM_SHMEMPAGES))); up_read(&mm->mmap_sem); - /* - * Drop our reference but make sure the mmput slow path is called from a - * different context because we shouldn't risk we get stuck there and - * put the oom_reaper out of the way. - */ - mmput_async(mm); trace_finish_task_reaping(tsk->pid); unlock_oom: mutex_unlock(&oom_lock); -- cgit v1.2.3