Re: [PATCH] modified memory_pressure calculation

[Manfred Spraul <manfred@colorfullife.com>]

Rik van Riel wrote:
> 
> On Sun, 27 May 2001, Manfred Spraul wrote:
> 
> > * if reclaim_page() finds a page that is Referenced, Dirty or Locked
> >   then it must increase memory_pressure.
> 
> Why ?
>
Because it means that a page that was not really unused was detected by
reclaim_page.
It's not really an increase of the memory pressure, it's a decrease of
the efficiency of the inactive_clean_list.

I'll think about it again.

> > * I don't understand the purpose of the second ++ in alloc_pages().
> 
> It's broken and should be removed. Thanks for spotting
> this one.
> 
> > What about the attached patch [vs. 2.4.5]? It's just an idea, untested.
> 
> Just remove the in_interrupt() check near PF_MEMALLOC, will you?
>
Of course.
I forgot to remove that line, it's unrelated to the memory_pressure
calculation. But I think it's another problem I spotted.

> Adding that check makes it possible for a pingflood to deadlock
> kswapd, as the network card can allocate the very last pages in
> the system and kswapd needs those pages to free up memory.
>

Are you sure? It should be the other way around:

>          if (z->free_pages < z->pages_min / 4 &&
> -           !(current->flags & PF_MEMALLOC))
> +            (in_interrupt() || !(current->flags & PF_MEMALLOC)))
>		continue;

It's 'if (in_interrupt()) continue', not 'if (in_interrupt()) alloc'.
Currently a network card can allocate the last few pages if the
interrupt occurs in the context of the PF_MEMALLOC thread. I think
PF_MEMALLOC memory should never be available to interrupt handlers.

--
	Manfred
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