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

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Date:	Sat, 31 May 2008 11:10:18 +0100
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Andrew Morton <akpm@...ux-foundation.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

Andrew Morton wrote:
> 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.
>   

OK, I'll respin with GFP_NOIO.

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

Well, if the allocation fails the machine is dead in the water, because 
it probably can't go on without its devices.  I think it's worth eating 
to the reserves to avoid that.  (I'll have a close look to only add it 
to allocations which are really unrecoverable if they fail.)

Thanks,
    J
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