linux-ext4 - Re: [PATCH v3 4/6] ext4: change lru to round-robin in extent status tree shrinker

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Message-ID: <20140903223805.GD12154@thunk.org>
Date:	Wed, 3 Sep 2014 18:38:05 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Jan Kara <jack@...e.cz>
Cc:	Zheng Liu <gnehzuil.liu@...il.com>, linux-ext4@...r.kernel.org,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Zheng Liu <wenqing.lz@...bao.com>
Subject: Re: [PATCH v3 4/6] ext4: change lru to round-robin in extent status
 tree shrinker

On Thu, Sep 04, 2014 at 12:14:02AM +0200, Jan Kara wrote:
> > I didn't think we were allowed to reschedule or sleep while in
> > shrinker context?
>   I believe we are allowed to sleep in the shrinker if appropriate gfp
> flags are set (__GFP_WAIT) and we enter extent cache shrinker only if
> __GFP_FS is set which guarantees __GFP_WAIT.

I must be missing something.  How is this guaranteed?

I can see how we can determine what gfp_mask was used in the
allocation which triggered the shrinker callback, by checking
shrink->gfp_mask, but I don't see anything that guarantees that the
extent cache shrinker is only entered if __GFP_FS is set.

I guess we could add something like:

	if ((shrink->gfp_mask & __GFP_WAIT) == 0)
		return 0;

to the beginning of ext4_es_scan(), but we're not doing that at the
moment.

       	     	      	      	      	 - Ted
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