linux-kernel - Re: [PATCH 2/5] oom reaper: handle mlocked pages

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Date:	Mon, 22 Feb 2016 17:36:07 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Michal Hocko <mhocko@...nel.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Andrea Argangeli <andrea@...nel.org>,
	Rik van Riel <riel@...hat.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 2/5] oom reaper: handle mlocked pages

On Wed, 3 Feb 2016, Michal Hocko wrote:

> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 9a0e4e5f50b4..840e03986497 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -443,13 +443,6 @@ static bool __oom_reap_vmas(struct mm_struct *mm)
>  			continue;
>  
>  		/*
> -		 * mlocked VMAs require explicit munlocking before unmap.
> -		 * Let's keep it simple here and skip such VMAs.
> -		 */
> -		if (vma->vm_flags & VM_LOCKED)
> -			continue;
> -
> -		/*
>  		 * Only anonymous pages have a good chance to be dropped
>  		 * without additional steps which we cannot afford as we
>  		 * are OOM already.
> @@ -459,9 +452,12 @@ static bool __oom_reap_vmas(struct mm_struct *mm)
>  		 * we do not want to block exit_mmap by keeping mm ref
>  		 * count elevated without a good reason.
>  		 */
> -		if (vma_is_anonymous(vma) || !(vma->vm_flags & VM_SHARED))
> +		if (vma_is_anonymous(vma) || !(vma->vm_flags & VM_SHARED)) {
> +			if (vma->vm_flags & VM_LOCKED)
> +				munlock_vma_pages_all(vma);
>  			unmap_page_range(&tlb, vma, vma->vm_start, vma->vm_end,
>  					 &details);
> +		}
>  	}
>  	tlb_finish_mmu(&tlb, 0, -1);
>  	up_read(&mm->mmap_sem);

Are we concerned about munlock_vma_pages_all() taking lock_page() and 
perhaps stalling forever, the same way it would stall in exit_mmap() for 
VM_LOCKED vmas, if another thread has locked the same page and is doing an 
allocation?  I'm wondering if in that case it would be better to do a 
best-effort munlock_vma_pages_all() with trylock_page() and just give up 
on releasing memory from that particular vma.  In that case, there may be 
other memory that can be freed with unmap_page_range() that would handle 
this livelock.