Re: [RFC] : mm : / Patch / code : Suggestion :snip kswapd &get_page_from_freelist() : No more no page failures. (WHY????)

[Nick Piggin <nickpiggin@yahoo.com.au>]

Mitchell Erblich wrote:
> Nick Piggin wrote:

> --------
> Nick Piggin, et al,
>
>     First diffs would generate alot of noise, since I rip and insert
>     alot of code based on whether I think the code is REALLY
>     needed for MY TEST environment. These suggestions are
>     basicly minimal merge suggestions between my
>     development envir and the public Linux tree.

That's OK. So long as the patch is against a well known tree, it
is just less ambiguous even if it doesn't actually compile :)


>     Now the why for this SUGGESTION/PATCH...
>
>
> > When we're in the (min,low) watermark range, we'll wake up kswapd
> > _before_ allocating anything, so what is better about the change to
> > wake up kswapd after allocating? Can you perhaps come up with an
> > example situation also to make this more clear?
>
>
> Answer
>     Will GFP_ATOMIC alloc be failing at that point? If yes, then why
>     not allow kswapd attempt to prevent this condition from occuring?
>     The existing code reads that the first call to get_page_from_freelist()
>     has returned no page. Now you are going to start up something that
>     is at best going to take millisecs to start helping out. Won't it first
>     grab some pages to do its work? So we are going to be lower
>     in free memory right when it starts up. Right?

GFP_ATOMIC will not be failing at this point (also, kswapd could
probably have reclaimed several hundred or thousand pages in 1ms,
but that's besides the point -- we do have correct buffering here).

The watermarks go roughly like this:

high -- kswapd stops reclaiming
low  -- kswapd is started by any allocation, nothing else happens
min  -- non-GFP_ATOMIC can't go below this point; enter direct reclaim
min/X-- GFP_ATOMIC allocations fail below this point
0    -- PF_MEMALLOC fails.

--
SUSE Labs, Novell Inc.
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