linux-kernel - Re: [PATCH] mm/vmscan: Do not block forever at shrink_inactive

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Message-Id: <20140519225915.3370328d.akpm@linux-foundation.org>
Date:	Mon, 19 May 2014 22:59:15 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>, riel@...hat.com,
	kosaki.motohiro@...fujitsu.com, fengguang.wu@...el.com,
	kamezawa.hiroyu@...fujitsu.com, linux-kernel@...r.kernel.org,
	xfs@....sgi.com
Subject: Re: [PATCH] mm/vmscan: Do not block forever at
 shrink_inactive_list().

On Tue, 20 May 2014 10:44:49 +1000 Dave Chinner <david@...morbit.com> wrote:

> @@ -258,14 +258,23 @@ xfs_bmapi_allocate_worker(
>  	struct xfs_bmalloca	*args = container_of(work,
>  						struct xfs_bmalloca, work);
>  	unsigned long		pflags;
> +	unsigned long		new_pflags = PF_FSTRANS;
>  
> -	/* we are in a transaction context here */
> -	current_set_flags_nested(&pflags, PF_FSTRANS);
> +	/*
> +	 * we are in a transaction context here, but may also be doing work
> +	 * in kswapd context, and hence we may need to inherit that state
> +	 * temporarily to ensure that we don't block waiting for memory reclaim
> +	 * in any way.
> +	 */
> +	if (args->kswapd)
> +		new_pflags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;

So current_is_kswapd() returns true for a thread which is not kswapd. 
That's a bit smelly.

Should this thread really be incrementing KSWAPD_INODESTEAL instead of
PGINODESTEAL, for example?  current_is_kswapd() does a range of things,
only one(?) of which you actually want.

It would be cleaner to create a new PF_ flag to select just that
behavior.  That's a better model than telling the world "I am magic and
special".

But we're awfully close to running out of PF_ space and I don't know if
this ugly justifies consuming a flag.

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