linux-kernel - Re: [patch 08/12] mm: page_alloc: wait for OOM killer progress before retrying

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Date:	Thu, 26 Mar 2015 16:38:47 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, ying.huang@...el.com,
	aarcange@...hat.com, david@...morbit.com, tytso@....edu
Subject: Re: [patch 08/12] mm: page_alloc: wait for OOM killer progress
 before retrying

On Thu 26-03-15 11:23:43, Johannes Weiner wrote:
> On Thu, Mar 26, 2015 at 03:32:23PM +0100, Michal Hocko wrote:
> > On Thu 26-03-15 07:24:45, Johannes Weiner wrote:
> > > On Wed, Mar 25, 2015 at 11:15:48PM +0900, Tetsuo Handa wrote:
> > > > Johannes Weiner wrote:
> > [...]
> > > > >  	/*
> > > > > -	 * Acquire the oom lock.  If that fails, somebody else is
> > > > > -	 * making progress for us.
> > > > > +	 * This allocating task can become the OOM victim itself at
> > > > > +	 * any point before acquiring the lock.  In that case, exit
> > > > > +	 * quickly and don't block on the lock held by another task
> > > > > +	 * waiting for us to exit.
> > > > >  	 */
> > > > > -	if (!mutex_trylock(&oom_lock)) {
> > > > > -		*did_some_progress = 1;
> > > > > -		schedule_timeout_uninterruptible(1);
> > > > > -		return NULL;
> > > > > +	if (test_thread_flag(TIF_MEMDIE) || mutex_lock_killable(&oom_lock)) {
> > > > > +		alloc_flags |= ALLOC_NO_WATERMARKS;
> > > > > +		goto alloc;
> > > > >  	}
> > > > 
> > > > When a thread group has 1000 threads and most of them are doing memory allocation
> > > > request, all of them will get fatal_signal_pending() == true when one of them are
> > > > chosen by OOM killer.
> > > > This code will allow most of them to access memory reserves, won't it?
> > > 
> > > Ah, good point!  Only TIF_MEMDIE should get reserve access, not just
> > > any dying thread.  Thanks, I'll fix it in v2.
> > 
> > Do you plan to post this v2 here for review?
> 
> Yeah, I was going to wait for feedback to settle before updating the
> code.  But I was thinking something like this?
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 9ce9c4c083a0..106793a75461 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2344,7 +2344,8 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order, int alloc_flags,
>  	 * waiting for us to exit.
>  	 */
>  	if (test_thread_flag(TIF_MEMDIE) || mutex_lock_killable(&oom_lock)) {
> -		alloc_flags |= ALLOC_NO_WATERMARKS;
> +		if (test_thread_flag(TIF_MEMDIE))
> +			alloc_flags |= ALLOC_NO_WATERMARKS;
>  		goto alloc;
>  	}

OK, I have expected something like this. I understand why you want to
retry inside this function. But I would prefer if gfp_to_alloc_flags was
used here so that we do not have that TIF_MEMDIE logic duplicated at two
places.
-- 
Michal Hocko
SUSE Labs
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