linux-kernel - Re: [PATCH v2 5/5] btrfs: enable swap file support

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Date:	Sat, 22 Nov 2014 12:03:57 -0800
From:	Omar Sandoval <osandov@...ndov.com>
To:	David Sterba <dsterba@...e.cz>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Mason <clm@...com>, Josef Bacik <jbacik@...com>,
	linux-btrfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-nfs@...r.kernel.org,
	Trond Myklebust <trond.myklebust@...marydata.com>,
	Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH v2 5/5] btrfs: enable swap file support

On Fri, Nov 21, 2014 at 07:00:45PM +0100, David Sterba wrote:
> > +			pr_err("BTRFS: swapfile has holes");
> > +			ret = -EINVAL;
> > +			goto out;
> > +		}
> > +		if (em->block_start == EXTENT_MAP_INLINE) {
> > +			pr_err("BTRFS: swapfile is inline");
> 
> While the test is valid, this would mean that the file is smaller than
> the inline limit, which is now one page. I think the generic swap code
> would refuse such a small file anyway.
> 
Sure. This test doesn't really cost us anything, so I think I'd feel a little
better just leaving it in. I'll add a comment for the next close reader.

Besides that and Filipe's response, I'll address everything you mentioned here
and in your other email in the next version, thanks.

> > +			ret = -EINVAL;
> > +			goto out;
> > +		}
> > +		if (test_bit(EXTENT_FLAG_COMPRESSED, &em->flags)) {
> > +			pr_err("BTRFS: swapfile is compresed");
> > +			ret = -EINVAL;
> > +			goto out;
> > +		}
> 
> I think the preallocated extents should be refused as well. This means
> the filesystem has enough space to hold the data but it would still have
> to go through the allocation and could in turn stress the memory
> management code that triggered the swapping activity in the first place.
> 
> Though it's probably still possible to reach such corner case even with
> fully allocated nodatacow file, this should be reviewed anyway.
> 
I'll definitely take a closer look at this. In particular,
btrfs_get_blocks_direct and btrfs_get_extent do allocations in some cases which
I'll look into.

-- 
Omar
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