From mboxrd@z Thu Jan  1 00:00:00 1970
Date: Tue, 9 Nov 2004 16:28:01 -0800
From: Andrew Morton <akpm@osdl.org>
Subject: Re: [PATCH] kswapd shall not sleep during page shortage
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To: Marcelo Tosatti <marcelo.tosatti@cyclades.com>
Cc: linux-mm@kvack.org, piggin@cyberone.com.au
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Marcelo Tosatti <marcelo.tosatti@cyclades.com> wrote:
>
> Back to arguing in favour of my patch - it seemed to me that kswapd could 
>  go to sleep leaving allocators which can't reclaim pages themselves in a 
>  bad situation. 

Yes, but those processes would be sleeping in blk_congestion_wait() during,
say, a GFP_NOIO/GFP_NOFS allocation attempt.  And in that case, they may be
holding locks whcih prevent kswapd from being able to do any work either.

>  It would have to be waken up by another instance of alloc_pages to then 
>  execute and start doing its job, while if it was executing already (madly 
>  scanning as you say), the chance it would find freeable pages quite
>  earlier.
> 
>  Note that not only disk IO can cause pages to become freeable. A user
>  can give up its reference on pagecache page for example (leaving
>  the page on LRU to be found and freed by kswapd).

yup.  Or munlock(), or direct-io completion.

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