linux-kernel - Re: [PATCH v2 -mm] make swapin readahead skip over holes

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Message-ID: <20120111214242.GF24386@cmpxchg.org>
Date:	Wed, 11 Jan 2012 22:42:42 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	akpm@...ux-foundation.org, mel@....ul.ie, minchan.kim@...il.com
Subject: Re: [PATCH v2 -mm] make swapin readahead skip over holes

On Wed, Jan 11, 2012 at 04:30:12PM -0500, Rik van Riel wrote:
> On 01/11/2012 04:10 PM, Johannes Weiner wrote:
> >On Wed, Jan 11, 2012 at 02:30:44PM -0500, Rik van Riel wrote:
> >>Ever since abandoning the virtual scan of processes, for scalability
> >>reasons, swap space has been a little more fragmented than before.
> >>This can lead to the situation where a large memory user is killed,
> >>swap space ends up full of "holes" and swapin readahead is totally
> >>ineffective.
> >>
> >>On my home system, after killing a leaky firefox it took over an
> >>hour to page just under 2GB of memory back in, slowing the virtual
> >>machines down to a crawl.
> >>
> >>This patch makes swapin readahead simply skip over holes, instead
> >>of stopping at them.  This allows the system to swap things back in
> >>at rates of several MB/second, instead of a few hundred kB/second.
> >>
> >>The checks done in valid_swaphandles are already done in
> >>read_swap_cache_async as well, allowing us to remove a fair amount
> >>of code.
> >
> >__swap_duplicate() also checks for whether the offset is within the
> >swap device range.  Do you think we could remove get_swap_cluster()
> >altogether and just try reading the aligned page_cluster range?
> 
> That is how I implemented it originally, but we need
> to take the swap_lock so it is cleaner to implement
> a helper function in swapfile.c :)

AFAICS, it's only needed to validate the offset against si->max, but
this too is done in __swap_duplicate().

What's otherwise left is just rounding down swp_offset(entry) and
adding 1 << page_cluster to it, that shouldn't need the swap_lock?

Am I missing something?
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