linux-kernel - Re: [PATCH 5 of 5] xen: Avoid allocations causing swap activity on the resume path

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Message-Id: <20080531025949.7c8b17bc.akpm@linux-foundation.org>
Date:	Sat, 31 May 2008 02:59:49 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Jeff Garzik <jgarzik@...ox.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ian Campbell <Ian.Campbell@...citrix.com>
Subject: Re: [PATCH 5 of 5] xen: Avoid allocations causing swap activity on
 the resume path

On Sat, 31 May 2008 10:50:40 +0100 Jeremy Fitzhardinge <jeremy@...p.org> wrote:

> Does __GFP_HIGH necessarily mean that it won't try to 
> do IO to push pages out?

Nope.

__GFP_FS: may enter filesystems
__GFP_IO: may perform IO

__GFP_IO also means "may do swapout".  Even when swap is on a regular
file.  This is because we do all the fs-related operations up-front
during swapon.  So at alloc_pages()-time we can go direct-to-disk-blocks.

So I assume for this application you'll need GFP_NOIO.  That's still
heaps better than GFP_ATOMIC, because it can sleep and wait for kswapd
to do stuff, and it can reclaim clean pagecache and clean swapcache.

Whether you should also add __GFP_HIGH to cause the page allocation to
bite harder into the page reserves is unclear to me, sorry.

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