Re: [2.4] heavy-load under swap space shortage

[Andrew Morton <akpm@osdl.org>]

Marcelo Tosatti <marcelo.tosatti@cyclades.com> wrote:
>
>  At first I was skeptic to the inclusion of this patch in v2.4 (due to the
>  freeze), but after thinking a bit more about I have a few points in favour 
>  of this modification (read Nomura's message below and the patch to know 
>  what I'm talking about):
> 
>  - It is off by default 
>  - It is very simple (non intrusive), it just changes the point in which 
>  anonymous pages are inserted into the LRU. 
>  - When turned on, I dont see it being a reason for introducing new 
>  bugs.
> 
>  What you think of this? 

hm, I hadn't noticed that 2.4 was changed to _not_ add anon pages to the
LRU.  I'd always regarded that as a workaround for pagemap_lru_lock contention
on large SMP machines which would never get beyond the suse kernel.

Having a magic knob is a weak solution: the majority of people who are
affected by this problem won't know to turn it on.

I confess that I don't really understand the failure mode.  So we have
zillions of anon pages which are not on the LRU.  We call swap_out() and
scan all these pages, failing to find swapcache space for them.

Why does adding the pages to the LRU up-front solve the problem?

(And why cannot we lazily add these anon pages to the LRU in swap_out, and
avoid the need for the knob?)