linux-ext4 - Re: [PATCH] jbd2: Fix unreclaimed pages after truncate in data=journal mode

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Date:	Thu, 26 Nov 2015 15:12:53 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-ext4@...r.kernel.org, Namjae Jeon <namjae.jeon@...sung.com>
Subject: Re: [PATCH] jbd2: Fix unreclaimed pages after truncate in
 data=journal mode

On Tue, Nov 24, 2015 at 09:47:31AM +0100, Jan Kara wrote:
> Ted and Namjae have reported that truncated pages don't get timely
> reclaimed after being truncated in data=journal mode. The following test
> triggers the issue easily:
> 
> for (i = 0; i < 1000; i++) {
> 	pwrite(fd, buf, 1024*1024, 0);
> 	fsync(fd);
> 	fsync(fd);
> 	ftruncate(fd, 0);
> }
> 
> The reason is that journal_unmap_buffer() finds that truncated buffers
> are not journalled (jh->b_transaction == NULL), they are part of
> checkpoint list of a transaction (jh->b_cp_transaction != NULL) and have
> been already written out (!buffer_dirty(bh)). We clean such buffers but
> we leave them in the checkpoint list. Since checkpoint transaction holds
> a reference to the journal head, these buffers cannot be released until
> the checkpoint transaction is cleaned up. And at that point we don't
> call release_buffer_page() anymore so pages detached from mapping are
> lingering in the system waiting for reclaim to find them and free them.
> 
> Fix the problem by removing buffers from transaction checkpoint lists
> when journal_unmap_buffer() finds out they don't have to be there
> anymore.
> 
> Reported-and-tested-by: Namjae Jeon <namjae.jeon@...sung.com>
> Fixes: de1b794130b130e77ffa975bb58cb843744f9ae5
> Signed-off-by: Jan Kara <jack@...e.cz>

Thanks, applied.  Many thanks to Namjae and Jan for tracking this down!!

		       	      	 	    - Ted
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