linux-kernel - Re: [PATCH] ipc, shm: make shmem attach/detach wait for mmap

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Date:	Fri, 11 Mar 2016 12:18:38 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Michal Hocko <mhocko@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Cc:	linux-mm@...ck.org, Davidlohr Bueso <dave@...olabs.net>,
	Michal Hocko <mhocko@...e.com>, Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH] ipc, shm: make shmem attach/detach wait for mmap_sem
 killable

On 03/09/2016 11:19 AM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
>
> shmat and shmdt rely on mmap_sem for write. If the waiting task
> gets killed by the oom killer it would block oom_reaper from
> asynchronous address space reclaim and reduce the chances of timely
> OOM resolving. Wait for the lock in the killable mode and return with
> EINTR if the task got killed while waiting.
>
> Cc: Hugh Dickins <hughd@...gle.com>
> Signed-off-by: Michal Hocko <mhocko@...e.com>
> Acked-by: Davidlohr Bueso <dave@...olabs.net>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>   ipc/shm.c | 9 +++++++--
>   1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/ipc/shm.c b/ipc/shm.c
> index 331fc1b0b3c7..13282510bc0d 100644
> --- a/ipc/shm.c
> +++ b/ipc/shm.c
> @@ -1200,7 +1200,11 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg, ulong *raddr,
>   	if (err)
>   		goto out_fput;
>
> -	down_write(&current->mm->mmap_sem);
> +	if (down_write_killable(&current->mm->mmap_sem)) {
> +		err = -EINTR;
> +		goto out_fput;
> +	}
> +
>   	if (addr && !(shmflg & SHM_REMAP)) {
>   		err = -EINVAL;
>   		if (addr + size < addr)
> @@ -1271,7 +1275,8 @@ SYSCALL_DEFINE1(shmdt, char __user *, shmaddr)
>   	if (addr & ~PAGE_MASK)
>   		return retval;
>
> -	down_write(&mm->mmap_sem);
> +	if (down_write_killable(&mm->mmap_sem))
> +		return -EINTR;
>
>   	/*
>   	 * This function tries to be smart and unmap shm segments that
>